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