HTML API: class_list yields lower case class names needs documentation

[jonsurrell]

That Tag Processor and HTML Processor yield lower case class names. This is by design, but it is undocumented.

Add documentation about this behavior.

The class_list method yields class name strings in lower case. This is
intended but undocumented. Document and add tests for this behavior.

Trac ticket: https://core.trac.wordpress.org/ticket/61520

[jonsurrell]

This is a documentation and test change that would be good to backport to 6.6 RC.

@sirreal changes LGTM.

@dmsnell Will you review these documentation changes? These would be good to backport to 6.6.

This may not be the best approach. Rather than documenting the specifics of the behavior here, it seems best to audit the class-related methods.

[jonsurrell]

Let's audit the behavior of class related behaviors instead. This current behavior may not be ideal. Superseded by #61531.