Documentation/branch: document -d --force and -m --force

The --force option was modified in 356e91f (branch: allow -f with -m and
-d, 2014-12-08), but the documentation was not updated.

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

Documentation/git-branch.txt

@@ -95,6 +95,10 @@ OPTIONS
 --force::
 	Reset <branchname> to <startpoint> if <branchname> exists
 	already. Without `-f` 'git branch' refuses to change an existing branch.
+	In combination with `-d` (or `--delete`), allow deleting the
+	branch irrespective of its merged status. In combination with
+	`-m` (or `--move`), allow renaming the branch even if the new
+	branch name already exists.
 
 -m::
 --move::