Introduce a "phriction.content.search" API method to replace "phriction.history"

Summary: Depends on D19096. Ref T13077. Adds a new "v3" API method for Phriction document content, to replace the existing "phriction.history" call.

Test Plan: Made various calls via web API console.

Maniphest Tasks: T13077

Differential Revision: https://secure.phabricator.com/D19097