PHPDoc: Replace "@return this" with "@return $this"

Summary:
Per https://github.com/php-fig/fig-standards/blob/master/proposed/phpdoc.md the valid keyword is $this.
Thus replace this to make output of static code analysis slightly less noisy.

(See rARC4b7ee1985b3c15b4af81b5480522ae24c0afd6a2 for its Arcanist counterpart.)

Test Plan: Read the docs.

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/D25819