Search - fix external redirect issue for "help" search

Summary: Fixes T7335. "help" gets you to a specific diviner doc which is an external link, so make sure the code sets is external for the redirect response in this case.

Test Plan: typed "help" and got some

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7335

Differential Revision: https://secure.phabricator.com/D11830