Change phpDoc return value of cast from Enumeration to static

Summary:
The PHPDoc of the method `Enumeration::cast()` has
the annotation `@return Enumeration` which shows a warning
in the IDE if you expect a specific Enumeration implementation.
This is fixed by changing the PHPDoc to `@return self`.

Test Plan: Check you IDE's warnings

Reviewers: claus.due

Reviewed By: claus.due

Differential Revision: https://phabricator.dkd.de/D184