Undeprecate I18N_NOOP2

Summary:
and document the pitfalls, where to use I18NC_NOOP, and where to use
I18N_NOOP2.

Test Plan: make test

Reviewers: ilic, aacid, mlaurent

Reviewed By: mlaurent

Subscribers: kossebau, aacid, vkrause, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D24884