Drop question mark suffix from optional PHPDoc @param types

Summary:
The question mark in @param type? $foo Desc is a custom notation not consistently applied across the codebase and not necessarily obvious to the reader (because custom and not mentioned in https://github.com/php-fig/fig-standards/blob/master/proposed/phpdoc.md ).
Instead, explicitly state "optional" in the parameter description for clarity.

Closes T15925.

Test Plan: Run PHPStan, see no PHPDoc @param has invalid value (type? [...] style output anymore.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15925

Differential Revision: https://we.phorge.it/D25797