Correct PHPDoc or add default return for numerous methods

Summary:
Correct PHPDoc when none of the method calls try to handle a return value.
Add missing fallback return statements (which may never be reached if previous condition checks were always true).

This reduces the amount of errors reported by static code analysis tools.

Test Plan: Carefully read the code and check all places in which a function gets called whether a return value is expected.

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/D25650