Improve atomizing of PHP @return annotation

Summary: Fixes T6890. This doesn't feel like a perfect solution, but I can't think of any cases in which this will produce the wrong result either.

Test Plan: Ran ./bin/diviner generate and checked the generated documentation for PhabricatorCommonPasswords::loadWordlist(). The return type was corrected shown as map<string, bool>.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6890

Differential Revision: https://secure.phabricator.com/D11469