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