Clarify documentation of multibyte-string-p.

src/data.c (Fmultibyte_string_p): Doc fix.

 doc/lispref/nonascii.texi (Text Representations): Document that
 multibyte-string-p returns nil for non-string objects.

doc/lispref/ChangeLog

@@ -1,3 +1,8 @@
+2013-07-06  Eli Zaretskii  <eliz@gnu.org>
+
+	* nonascii.texi (Text Representations): Document that
+	multibyte-string-p returns nil for non-string objects.
+
 2013-07-06  Glenn Morris  <rgm@gnu.org>
 
 	* elisp.texi (Top): Move WWW_GNU_ORG section outside @copying.

doc/lispref/nonascii.texi

@@ -124,7 +124,8 @@ belong to the same character.
 
 @defun multibyte-string-p string
 Return @code{t} if @var{string} is a multibyte string, @code{nil}
-otherwise.
+otherwise.  This function also returns @code{nil} if @var{string} is
+some object other than a string.
 @end defun
 
 @defun string-bytes string

src/ChangeLog

@@ -1,3 +1,7 @@
+2013-07-06  Eli Zaretskii  <eliz@gnu.org>
+
+	* data.c (Fmultibyte_string_p): Doc fix.
+
 2013-07-05  Paul Eggert  <eggert@cs.ucla.edu>
 
 	Use emacs_open more consistently when opening files.

src/data.c

@@ -377,7 +377,8 @@ DEFUN ("stringp", Fstringp, Sstringp, 1, 1, 0,
 
 DEFUN ("multibyte-string-p", Fmultibyte_string_p, Smultibyte_string_p,
        1, 1, 0,
-       doc: /* Return t if OBJECT is a multibyte string.  */)
+       doc: /* Return t if OBJECT is a multibyte string.
+Return nil if OBJECT is either a unibyte string, or not a string.  */)
   (Lisp_Object object)
 {
   if (STRINGP (object) && STRING_MULTIBYTE (object))