[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