ls-files documentation: reword for consistency
ls-files documentation: reword for consistency

Similar to descriptions of other options, state what -x does in imperative
mood.  Start sentences for -X and --exclude-per-directory options in
capital letters.

Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

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