R1:68486c4541dd

Rename `PhabricatorSearchField` to `PhabricatorSearchFieldDocumentType`

Summary: Ref T8441. I want to use `PhabricatorSearchField` for a better, more useful object.

Test Plan: `grep`, `arc lint`

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8441

Differential Revision: https://secure.phabricator.com/D13168