Document string-prefix-p

* doc/lispref/strings.texi (Text Comparison): Mention string-prefix-p.
* etc/NEWS: Belated entry.

doc/lispref/ChangeLog

+2012-03-22  Glenn Morris  <rgm@gnu.org>
+
+	* strings.texi (Text Comparison): Mention string-prefix-p.
+
 2012-03-21  Chong Yidong  <cyd@gnu.org>
 
 	* display.texi (The Echo Area): Add xref to Output Streams.

doc/lispref/strings.texi

@@ -508,6 +508,13 @@ are used.
 @code{string-lessp} is another name for @code{string<}.
 @end defun
 
+@defun string-prefix-p string1 string2 &optional ignore-case
+This function returns non-@code{nil} if @var{string1} is a prefix of
+@var{string2}; i.e., if @var{string2} starts with @var{string1}.  If
+the optional argument @var{ignore-case} is non-@code{nil}, the
+comparison ignores case differences.
+@end defun
+
 @defun compare-strings string1 start1 end1 string2 start2 end2 &optional ignore-case
 This function compares the specified part of @var{string1} with the
 specified part of @var{string2}.  The specified part of @var{string1}

etc/NEWS

@@ -1547,6 +1547,10 @@ from multiple input maps.  You can use this to make a keymap that
 inherits from multiple maps, eg:
  (set-keymap-parent newmap (make-composed-keymap othermap parent))
 
++++
+** New function `string-prefix-p'.
+(This was actually added in Emacs 23.2 but was not advertised at the time.)
+
 +++
 ** New reader macro ## that stands for the empty symbol.
 This means that the empty symbol can now be read back.  Also, #: by itself