git-cl: remove documentation regarding git diff args

R=tandrii

Bug: 768445
Change-Id: I04605b3e95b08d71aedde23c97b6ca985f030eb6
Reviewed-on: https://chromium-review.googlesource.com/753131
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Aaron Gable <agable@chromium.org>

git_cl.py

@@ -4865,7 +4865,7 @@ def cleanup_list(l):
   return sorted(filter(None, stripped_items))
 
 
-@subcommand.usage('[args to "git diff"]')
+@subcommand.usage('[flags]')
 def CMDupload(parser, args):
   """Uploads the current changelist to codereview.