[libkwineffects] Overhaul AnimationEffect's documentation
Summary: This change reformats the documentation so it's more easier to read it.
Test Plan:
Generated Doxygen documentation, started Python's Simple HTTP server, navigated
to AnimationEffect's documention, haven't noticed any serious issues.
Reviewers: KWin, davidedmundson
Reviewed By: KWin, davidedmundson
Subscribers: graesslin, davidedmundson, kwin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D17832