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