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: {F154209}

Reviewers: lpriestley

Reviewed By: lpriestley

Subscribers: epriestley

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

src/applications/passphrase/controller/PassphraseCredentialEditController.php

@@ -293,7 +293,11 @@ final class PassphraseCredentialEditController extends PassphraseController {
           ->addCheckbox(
             'lock',
             1,
-            pht('Lock Permanently'),
+            array(
+              phutil_tag('strong', array(), pht('Lock Permanently:')),
+              ' ',
+              pht('Prevent the secret from being revealed or changed.'),
+            ),
             $v_is_locked)
           ->setDisabled($credential_is_locked));
     }