Add documentation to the PHPUnit factory objects

[andizer]

I discovered that the php unit test factories for WordPress aren't documented well. To make it easier for every one I decided to add some documentation for it.

Another thing I was wondering about is the old php4 way to declare methods (without access modifiers) and var declarations on class attributes. Is it desirable to modernize these or should I keep this untouched. Probably it is worth to make a separate issue for it.

[JPry]

@andizer

One thing I noticed when doing a quick skim of your patch: The docblock for WP_UnitTest_Factory_Callback_After_Create::__construct() has string as the parameter type. This should actually be callable. While this type hint is only valid in PHP 5.4+, using it in the docblock won't have any issue.

[andizer]

@JPry Thanks. You are right, it makes more sense to have it named callable. I've changed it.

[pento]

In 44497:

Docs: Add docblocks for the PHPUnit factory objects.

Props andizer.
Fixes #44521.