Misc PHPDoc additions to PhabricatorFile

Summary: Add some PHPDoc comments to the PhabricatorFile class as I was playing with calling some of its methods.

Test Plan: Call those methods and see what they do and return.

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25763