Update git-grep documentation
Update git-grep documentation

Documentation/git-grep.txt: Document -F/--fixed-strings to
search for non-regexp patterns.  Document -I to not search
binary files.  Document -<num> as a shortcut for -C<num>.

Signed-off-by: Andrew Ruder <e3579b1e47f273529f0f929453e939a68ede9fd1@aeruder.net>
Signed-off-by: Junio C Hamano <dc50d1021234060e53ec42a77d526afa2fe07479@cox.net>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git