Improve documentation of TextEntity stuff

Summary:
This adds some important documentation on TextEntity and other classes, and improves some of the existing documentation.

This includes changing parameter names from ‘rect’ to ‘area’, because I found ‘rect’ misleading.

Test Plan: Run doxygen

Reviewers: Okular, aacid

Reviewed By: Okular, aacid

Subscribers: aacid, yurchor, okular-devel

Tags: Okular

Differential Revision: https://phabricator.kde.org/D21271