Correct PHPDoc return value for loadObject() in PhabricatorObjectMailReceiver

Summary:
The PHPDoc for the abstract function `loadObject()` defines `@return void`, however the function's return value is used twice in `PhabricatorObjectMailReceiver` and all 17 `loadObject()` implementations in child classes return an object provided by some type of `PhabricatorCursorPagedPolicyAwareQuery`.
Thus correct the PHPDoc to make static code analyzers happier.

Test Plan: Carefully read and compare the code.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25654

src/applications/metamta/receiver/PhabricatorObjectMailReceiver.php

@@ -22,7 +22,8 @@ abstract class PhabricatorObjectMailReceiver extends PhabricatorMailReceiver {
    * @param   string          A string matched by @{method:getObjectPattern}
    *                          fragment.
    * @param   PhabricatorUser The viewing user.
-   * @return  void
+   * @return  object|null     The object to receive mail, or null if no such
+   *                          object exists.
    */
   abstract protected function loadObject($pattern, PhabricatorUser $viewer);