checkout: refer to other-worktree branch, not ref

We can only check out commits or branches, not refs in general.  And the
problem here is if another worktree is using the branch that we want to
check out.

Let’s be more direct and just talk about branches instead of refs.

Also replace “be held” with “in use”.  Further, “in use” is not
restricted to a branch being checked out (e.g. the branch could be busy
on a rebase), hence generalize to “or otherwise in use” in the option
description.

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-checkout.txt

@@ -290,10 +290,10 @@ Note that this option uses the no overlay mode by default (see also
 `--overlay`), and currently doesn't support overlay mode.
 
 --ignore-other-worktrees::
-	`git checkout` refuses when the wanted ref is already checked
-	out by another worktree. This option makes it check the ref
-	out anyway. In other words, the ref can be held by more than one
-	worktree.
+	`git checkout` refuses when the wanted branch is already checked
+	out or otherwise in use by another worktree. This option makes
+	it check the branch out anyway. In other words, the branch can
+	be in use by more than one worktree.
 
 --overwrite-ignore::
 --no-overwrite-ignore::

builtin/checkout.c

@@ -1716,7 +1716,7 @@ static struct option *add_common_switch_branch_options(
 			   N_("update ignored files (default)"),
 			   PARSE_OPT_NOCOMPLETE),
 		OPT_BOOL(0, "ignore-other-worktrees", &opts->ignore_other_worktrees,
-			 N_("do not check if another worktree is holding the given ref")),
+			 N_("do not check if another worktree is using this branch")),
 		OPT_END()
 	};
 	struct option *newopts = parse_options_concat(prevopts, options);