Expand documentation for "transaction.search"

Summary: Depends on D20209. Ref T13255. It would probably be nice to make this into a "real" *.search API method some day, but at least document the features for now.

Test Plan: Read documentation.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13255

Differential Revision: https://secure.phabricator.com/D20211