Remove doxygen docs of internal class

Summary:
And copy the relevant snippet to the relevant public class.

I tried writing a Kirigami app over the weekend and more time than I'd
like to admit wondering why I couldn't use this documented class.

Test Plan: NA

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D13476