Provide more context on what "Lock Permanently" means in the UI

Summary:
Administrators may encounter this UI for the first time when configuring a repository, without ever having used Passphrase before. If they end up here on a workflow like this, the meaning of "Lock Permanently" may not be totally clear. Add some explanatory text.

(See also T4721, which discusses Passphrase documentation more generally.)

Test Plan:

Reviewers: lpriestley

Reviewed By: lpriestley

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D9102