Improve documentation of area classes

Summary:
This shall improve the documentation of several area classes,
including NormalizedPoint, NormalizedRect, RegularArea, RegularAreaRect.

This shall also clarify when absolute coordinates and when normalized
coordinates are used.

Describes the normalized coordinate system in NormalizedPoint, with a new term “reference area” do describe mapping.

TODO:
Page view rotation.
This is not done consistently in Okular, but can be changed later. I think this documentation will help (me) with that then.

Test Plan: Run doxygen

Reviewers: #okular, aacid

Reviewed By: #okular, aacid

Subscribers: aacid, okular-devel

Tags: #okular

Differential Revision: https://phabricator.kde.org/D21266