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>

Documentation/git-branch.txt

@@ -81,7 +81,7 @@ OPTIONS
 	`--track` or `--set-upstream`.
 
 -D::
-	Delete a branch irrespective of its merged status.
+	Shortcut for `--delete --force`.
 
 -l::
 --create-reflog::
@@ -105,7 +105,7 @@ OPTIONS
 	Move/rename a branch and the corresponding reflog.
 
 -M::
-	Move/rename a branch even if the new branch name already exists.
+	Shortcut for `--move --force`.
 
 --color[=<when>]::
 	Color branches to highlight current, local, and