Facts and Charts: Add more PHPDoc coverage

Summary: Make things a bit easier to understand.

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

Reviewers: O1 Blessed Committers, Cigaryno, valerio.bozzolan

Reviewed By: O1 Blessed Committers, Cigaryno, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25906