[KCMs] Use consistent text for GHNS buttons
Summary:
Implement consistent text format as decided on in T9941. Also change docbooks, as appropriate.
Closes T9941
Test Plan:
Subscribers: broulik, Codezela, abetts, plasma-devel, kde-doc-english
Tags: Plasma, Documentation
Maniphest Tasks: T9941
Differential Revision: https://phabricator.kde.org/D16504