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