stash doc: add a warning about using create

Add a note saying that the user probably wants "save" in the create
description.  While at it, document that it can optionally take a
message in the synopsis.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-stash.txt

@@ -16,7 +16,7 @@ SYNOPSIS
 'git stash' [save [--patch] [-k|--[no-]keep-index] [-q|--quiet]
 	     [-u|--include-untracked] [-a|--all] [<message>]]
 'git stash' clear
-'git stash' create
+'git stash' create [<message>]
 
 DESCRIPTION
 -----------
@@ -151,6 +151,8 @@ create::
 
 	Create a stash (which is a regular commit object) and return its
 	object name, without storing it anywhere in the ref namespace.
+	This is intended to be useful for scripts.  It is probably not
+	the command you want to use; see "save" above.
 
 
 DISCUSSION