Clarify language when Autoclose is disabled for a repository

Summary: Fixes T12051, adds additional language.

Test Plan:
Disable Autoclose in Actions, see updated language under Branches.

{F5147291}

Reviewers: epriestley

Reviewed By: epriestley

Spies: Korvin

Maniphest Tasks: T12051

Differential Revision: https://secure.phabricator.com/D18482