Documentation: --amend cannot be combined with -c/-C/-F.

We used to get the following confusing error message:

    $ git commit --amend -a -m foo
    Option -m cannot be combined with -c/-C/-F

This is because --amend cannot be combined with -c/-C/-F, which makes
sense, because they try to handle the same log message in different ways.
So update the documentation to reflect this.
Signed-off-by: NPeter Eriksen <s022018@student.dtu.dk>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

Documentation/git-commit.txt

@@ -8,8 +8,8 @@ git-commit - Record changes to the repository
 SYNOPSIS
 --------
 [verse]
-'git-commit' [-a] [-s] [-v] [(-c | -C) <commit> | -F <file> | -m <msg>]
-	   [--no-verify] [--amend] [-e] [--author <author>]
+'git-commit' [-a] [-s] [-v] [(-c | -C) <commit> | -F <file> | -m <msg> |
+	    --amend] [--no-verify] [-e] [--author <author>]
 	   [--] [[-i | -o ]<file>...]
 
 DESCRIPTION

git-commit.sh

@@ -3,7 +3,7 @@
 # Copyright (c) 2005 Linus Torvalds
 # Copyright (c) 2006 Junio C Hamano
 
-USAGE='[-a] [-s] [-v] [--no-verify] [-m <message> | -F <logfile> | (-C|-c) <commit>] [-u] [--amend] [-e] [--author <author>] [[-i | -o] <path>...]'
+USAGE='[-a] [-s] [-v] [--no-verify] [-m <message> | -F <logfile> | (-C|-c) <commit> | --amend] [-u] [-e] [--author <author>] [[-i | -o] <path>...]'
 SUBDIRECTORY_OK=Yes
 . git-sh-setup
 require_work_tree
@@ -284,9 +284,9 @@ esac
 
 case "$log_given" in
 tt*)
-	die "Only one of -c/-C/-F can be used." ;;
+	die "Only one of -c/-C/-F/--amend can be used." ;;
 *tm*|*mt*)
-	die "Option -m cannot be combined with -c/-C/-F." ;;
+	die "Option -m cannot be combined with -c/-C/-F/--amend." ;;
 esac
 
 case "$#,$also,$only,$amend" in