Overhaul doxygen comments

Summary:
We have a mix of different doxygen comment styles, e.g.

    /*!
      Foo bar.
     */

    /**
     * Foo bar.
     */

    /** Foo bar.
     */

    /**
     * Foo bar.
     */

    /**
     * Foo bar.
     **/

To make the code more consistent, this change updates the style of all
doxygen comments to the last one.

Test Plan: Compiles.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D18683