completion: add a note that merge options are shared

This should avoid future confusion after a subsequent patch has added
some options to __git_merge_options and some directly in _git_merge().

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1472,6 +1472,7 @@ _git_log ()
 	__git_complete_revlist
 }
 
+# Common merge options shared by git-merge(1) and git-pull(1).
 __git_merge_options="
 	--no-commit --no-stat --log --no-log --squash --strategy
 	--commit --stat --no-squash --ff --no-ff --ff-only --edit --no-edit