Documentation: --amend cannot be combined with -c/-C/-F.

Documentation: --amend cannot be combined with -c/-C/-F.

We used to get the following confusing error message:

    $ git commit --amend -a -m foo
    Option -m cannot be combined with -c/-C/-F

This is because --amend cannot be combined with -c/-C/-F, which makes
sense, because they try to handle the same log message in different ways.
So update the documentation to reflect this.

Signed-off-by: Peter Eriksen <s022018@student.dtu.dk>
Signed-off-by: Junio C Hamano <junkio@cox.net>