Documentation/branch: document -M and -D in terms of --force

Documentation/branch: document -M and -D in terms of --force

Now that we have proper documentation for --force's interaction with -d
and -m, we can avoid duplication and consider -M and -D as convenience
aliases for -m --force and -d --force.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>