Clarify use of the term "scanning" in the manual

* doc/ref/api-memory.texi (Garbage Collection Functions):
* doc/ref/libguile-concepts.texi (Garbage Collection): Attempt to be
  clear that scanning is a thing that happens in the mark phase.  Fixes
  #20907 I think.

doc/ref/api-memory.texi

@@ -27,9 +27,10 @@ collection relates to using Guile from C.
 
 @deffn {Scheme Procedure} gc
 @deffnx {C Function} scm_gc ()
-Scans all of SCM objects and reclaims for further use those that are
+Finds all of the ``live'' @code{SCM} objects and reclaims for further
-no longer accessible.  You normally don't need to call this function
+use those that are no longer accessible.  You normally don't need to
-explicitly.  It is called automatically when appropriate.
+call this function explicitly.  Its functionality is invoked
+automatically as needed.
 @end deffn
 
 @deftypefn {C Function} SCM scm_gc_protect_object (SCM @var{obj})
@@ -43,8 +44,9 @@ than it has been protected. Returns the SCM object it was passed.
 
 Note that storing @var{obj} in a C global variable has the same
 effect@footnote{In Guile up to version 1.8, C global variables were not
-scanned by the garbage collector; hence, @code{scm_gc_protect_object}
+visited by the garbage collector in the mark phase; hence,
-was the only way in C to prevent a Scheme object from being freed.}.
+@code{scm_gc_protect_object} was the only way in C to prevent a Scheme
+object from being freed.}.
 @end deftypefn
 
 @deftypefn {C Function} SCM scm_gc_unprotect_object (SCM @var{obj})
@@ -123,16 +125,18 @@ live reference to it@footnote{In Guile up to version 1.8, memory
 allocated with @code{scm_gc_malloc} @emph{had} to be freed with
 @code{scm_gc_free}.}.
 
-Memory allocated with @code{scm_gc_malloc} is scanned for live pointers.
+When garbage collection occurs, Guile will visit the words in memory
-This means that if @code{scm_gc_malloc}-allocated memory contains a
+allocated with @code{scm_gc_malloc}, looking for live pointers.  This
-pointer to some other part of the memory, the garbage collector notices
+means that if @code{scm_gc_malloc}-allocated memory contains a pointer
-it and prevents it from being reclaimed@footnote{In Guile up to 1.8,
+to some other part of the memory, the garbage collector notices it and
-memory allocated with @code{scm_gc_malloc} was @emph{not} scanned.
+prevents it from being reclaimed@footnote{In Guile up to 1.8, memory
-Consequently, the GC had to be told explicitly about pointers to live
+allocated with @code{scm_gc_malloc} was @emph{not} visited by the
-objects contained in the memory block, e.g., @i{via} SMOB mark functions
+collector in the mark phase.  Consequently, the GC had to be told
-(@pxref{Smobs, @code{scm_set_smob_mark}})}.  Conversely, memory
+explicitly about pointers to live objects contained in the memory block,
-allocated with @code{scm_gc_malloc_pointerless} is assumed to be
+e.g., @i{via} SMOB mark functions (@pxref{Smobs,
-``pointer-less'' and is not scanned.
+@code{scm_set_smob_mark}})}.  Conversely, memory allocated with
+@code{scm_gc_malloc_pointerless} is assumed to be ``pointer-less'' and
+is not scanned for pointers.
 
 For memory that is not associated with a Scheme object, you can use
 @code{scm_malloc} instead of @code{malloc}.  Like
@@ -193,9 +197,11 @@ Allocate @var{size} bytes of automatically-managed memory.  The memory
 is automatically freed when no longer referenced from any live memory
 block.
 
-Memory allocated with @code{scm_gc_malloc} or @code{scm_gc_calloc} is
+When garbage collection occurs, Guile will visit the words in memory
-scanned for pointers.  Memory allocated by
+allocated with @code{scm_gc_malloc} or @code{scm_gc_calloc}, looking for
-@code{scm_gc_malloc_pointerless} is not scanned.
+pointers to other memory allocations that are managed by the GC.  In
+contrast, memory allocated by @code{scm_gc_malloc_pointerless} is not
+scanned for pointers.
 
 The @code{scm_gc_realloc} call preserves the ``pointerlessness'' of the
 memory area pointed to by @var{mem}.  Note that you need to pass the old

doc/ref/libguile-concepts.texi

@@ -203,22 +203,38 @@ set'' of garbage collection; any value on the heap that is referenced
 directly or indirectly by a member of the root set is preserved, and all
 other objects are eligible for reclamation.
 
-The Scheme stack and heap are scanned precisely; that is to say, Guile
+In Guile, garbage collection has two logical phases: the @dfn{mark
-knows about all inter-object pointers on the Scheme stack and heap.
+phase}, in which the collector discovers the set of all live objects,
-This is not the case, unfortunately, for pointers on the C stack and
+and the @dfn{sweep phase}, in which the collector reclaims the resources
-static data segment.  For this reason we have to scan the C stack and
+associated with dead objects.  The mark phase pauses the program and
-static data segment @dfn{conservatively}; any value that looks like a
+traces all @code{SCM} object references, starting with the root set.
-pointer to a GC-managed object is treated as such, whether it actually
+The sweep phase actually runs concurrently with the main program,
-is a reference or not.  Thus, scanning the C stack and static data
+incrementally reclaiming memory as needed by allocation.
-segment is guaranteed to find all actual references, but it might also
+
-find words that only accidentally look like references.  These ``false
+In the mark phase, the garbage collector traces the Scheme stack and
-positives'' might keep @code{SCM} objects alive that would otherwise be
+heap @dfn{precisely}.  Because the Scheme stack and heap are managed by
-considered dead.  While this might waste memory, keeping an object
+Guile, Guile can know precisely where in those data structures it might
-around longer than it strictly needs to is harmless.  This is why this
+find references to other heap objects.  This is not the case,
-technique is called ``conservative garbage collection''.  In practice,
+unfortunately, for pointers on the C stack and static data segment.
-the wasted memory seems to be no problem, as the static C root set is
+Instead of requiring the user to inform Guile about all variables in C
-almost always finite and small, given that the Scheme stack is separate
+that might point to heap objects, Guile traces the C stack and static
-from the C stack.
+data segment @dfn{conservatively}.  That is to say, Guile just treats
+every word on the C stack and every C global variable as a potential
+reference in to the Scheme heap@footnote{Note that Guile does not scan
+the C heap for references, so a reference to a @code{SCM} object from a
+memory segment allocated with @code{malloc} will have to use some other
+means to keep the @code{SCM} object alive.  @xref{Garbage Collection
+Functions}.}.  Any value that looks like a pointer to a GC-managed
+object is treated as such, whether it actually is a reference or not.
+Thus, scanning the C stack and static data segment is guaranteed to find
+all actual references, but it might also find words that only
+accidentally look like references.  These ``false positives'' might keep
+@code{SCM} objects alive that would otherwise be considered dead.  While
+this might waste memory, keeping an object around longer than it
+strictly needs to is harmless.  This is why this technique is called
+``conservative garbage collection''.  In practice, the wasted memory
+seems to be no problem, as the static C root set is almost always finite
+and small, given that the Scheme stack is separate from the C stack.
 
 The stack of every thread is scanned in this way and the registers of
 the CPU and all other memory locations where local variables or function