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