R1:5f565e2ab665

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.