David Hurka authored Jul 21, 2019 and Albert Astals Cid committed Jul 21, 2019

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