Improve documentation

Summary:
Some of the documentation did not say what the purpose of the
respective class was, but rather gave a tautology.
For people new to the code base it helps to mention that
DPMS is power management.

That DataDevice is for copy-and-paste and drag-and-drop is
maybe obvious to people used to X11 API, but not very clear
to most others.

When first reading the KWayland code, some explanations
help getting around.

Reviewers: KWin, romangg

Reviewed By: KWin, romangg

Subscribers: romangg, zzag, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D23915