ManiphestReportController: Add more PHPDoc coverage

Summary: Make things a bit easier to understand, for the start.

Test Plan: Check return values for their types and classes.

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/D25901