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

src/applications/diviner/atomizer/DivinerPHPAtomizer.php

@@ -293,7 +293,7 @@ final class DivinerPHPAtomizer extends DivinerAtomizer {
           'this implicitly.');
       }
     } else if ($return) {
-      $split = preg_split('/\s+/', trim($return), $limit = 2);
+      $split = preg_split('/(?<!,)\s+/', trim($return), $limit = 2);
       if (!empty($split[0])) {
         $type = $split[0];
       }