Improve documentation of Document::setPageTextSelection()

Summary:
Improve the documentation of Document::setPageTextSelection()
and PagePrivate::setTextSelections(). Also mention which argument
will be deleted.

Test Plan: Check doxygen output

Reviewers: #okular, aacid

Reviewed By: #okular, aacid

Subscribers: okular-devel

Tags: #okular

Differential Revision: https://phabricator.kde.org/D21069