grep doc: add --break / --heading / -W to synopsis

All of the other options were included in the synopsis, so it makes
sense to include these as well.

Signed-off-by: Mark Lodato <lodatom@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-grep.txt

@@ -20,7 +20,9 @@ SYNOPSIS
 	   [-c | --count] [--all-match] [-q | --quiet]
 	   [--max-depth <depth>]
 	   [--color[=<when>] | --no-color]
+	   [--break] [--heading] [-p | --show-function]
 	   [-A <post-context>] [-B <pre-context>] [-C <context>]
+	   [-W | --function-context]
 	   [-f <file>] [-e] <pattern>
 	   [--and|--or|--not|(|)|-e <pattern>...]
 	   [ [--exclude-standard] [--cached | --no-index | --untracked] | <tree>...]