(SLIB): Add notes on delete-file, provided? and open-file

overridden by ice-9 slib module.

doc/ref/slib.texi

@@ -27,11 +27,41 @@ slib, The SLIB Manual}).  For example,
 @result{} #t
 @end example
 
+Note that the following Guile core functions are overridden by
+@code{(ice-9 slib)}, to implement SLIB specified semantics.
+
+@table @code
+@item delete-file
+@findex delete-file
+Returns @code{#t} for success or @code{#f} for failure
+(@pxref{Input/Output,,, slib, The SLIB Manual}), as opposed to the
+Guile core version unspecified for success and throwing an error for
+failure (@pxref{File System}).
+
+@c  `provide' is also exported by ice-9 slib, but its definition in
+@c  slib require.scm is the same as guile boot-9.scm, so believe
+@c  nothing needs to be said about that.
+
+@item provided?
+@findex provided?
+Accepts a feature specification containing @code{and} and @code{or}
+forms combining symbols (@pxref{Feature,,, slib, The SLIB Manual}), as
+opposed to the Guile core taking only plain symbols (@pxref{Feature
+Manipulation}).
+
+@item open-file
+@findex open-file
+Takes a symbol @code{r}, @code{rb}, @code{w} or @code{wb} for the open
+mode (@pxref{Input/Output,,, slib, The SLIB Manual}), as opposed to
+the Guile core version taking a string (@pxref{File Ports}).
+
+@item system
 @findex system
-Note that @code{(ice-9 slib)} provides a new definition of
-@code{system}, one giving a plain exit code return value, as per the
-SLIB specification (@pxref{System Interface,,, slib, The SLIB
-Manual}).
+Returns a plain exit code 0 to 255 (@pxref{System Interface,,, slib,
+The SLIB Manual}), as opposed to the Guile core version returning a
+wait status that must be examined with @code{status:exit-val} etc
+(@pxref{Processes}).
+@end table
 
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