branch: document -f and linked worktree behaviour

"git branch -f name start" forces to recreate the named branch, but
the forcing does not defeat the "do not touch a branch that is
checked out elsewhere" safety valve.

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

Documentation/git-branch.txt

@@ -123,6 +123,10 @@ OPTIONS
 	points to a valid commit. In combination with
 	`-m` (or `--move`), allow renaming the branch even if the new
 	branch name already exists, the same applies for `-c` (or `--copy`).
++
+Note that 'git branch -f <branchname> [<start-point>]', even with '-f',
+refuses to change an existing branch `<branchname>` that is checked out
+in another worktree linked to the same repository.
 
 -m::
 --move::