rP123831b53fb7

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? [...]`…