git-pull documentation: warn about the option order

We might eventually be loosening this rule, but there is a longstanding
restriction that the users currently need to be aware of.

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

Documentation/git-pull.txt

@@ -20,6 +20,8 @@ Note that you can use `.` (current directory) as the
 <repository> to pull from the local repository -- this is useful
 when merging local branches into the current branch.
 
+Also note that options meant for `git-pull` itself and underlying
+`git-merge` must be given before the options meant for `git-fetch`.
 
 OPTIONS
 -------