Use more traditional doxygen style

Summary:
So far we were following a bit unique and rare doxygen comment style:

    /**
     * Contents of the comment.
     **/

Doxygen comments with this style look balanced and neat, but many people
that contribute to KWin don't follow this style. Instead, they prefer
more traditional doxygen comment style, i.e.

    /**
     * Contents of the comment.
     */

Reviewing such changes has been a bit frustrating for me (so selfish!)
and for other contributors.

This change switches doxygen comment style in KWin to a more traditional
style. The main reason for doing this is to make code review process easier
for new contributors as well us.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D22812