Clarify --debug' vs. --no-debug'.

* module/ice-9/command-line.scm (*usage*): Attempt to suggest that
  `--no-debug' doesn't inhibit debugging support.

* doc/ref/guile-invoke.texi (Command-line Options): Make it clear that
  `--no-debug' doesn't inhibit debugging support.  Reported by Manuel
  Serrano.

doc/ref/guile-invoke.texi

@@ -156,6 +156,12 @@ interactive session.  When executing a script with @option{-s} or
 Do not use the debugging VM engine, even when entering an interactive
 session.
 
+Note that, despite the name, Guile running with @option{--no-debug}
+@emph{does} support the usual debugging facilities, such as printing a
+detailed backtrace upon error.  The only difference with
+@option{--debug} is lack of support for VM hooks and the facilities that
+build upon it (see above).
+
 @item -q
 @cindex init file, not loading
 @cindex @file{.guile} file, not loading

module/ice-9/command-line.scm

@@ -122,8 +122,8 @@ If FILE begins with `-' the -s switch is mandatory.
   -e FUNCTION    after reading script, apply FUNCTION to
                  command line arguments
   -ds            do -s script at this point
-  --debug        start with debugging evaluator and backtraces
+  --debug        start with the \"debugging\" VM engine
-  --no-debug     start with normal evaluator
+  --no-debug     start with the normal VM engine, which also supports debugging
                  Default is to enable debugging for interactive
                  use, but not for `-s' and `-c'.
   --auto-compile compile source files automatically