Correct docs of PhabricatorPHIDType::getAllTypesForApplication()

Summary: The signature and return type of the getAllTypesForApplication() function differed from its docs.

Test Plan: Carefully read the code of the function and compare it to the getAllInstalledTypes() function above from which the docs were incorrectly copied.

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/D25572