[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:
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Reviewers: #plasma, #vdg

Subscribers: broulik, Codezela, abetts, plasma-devel, kde-doc-english

Tags: #plasma, #documentation

Maniphest Tasks: T9941

Differential Revision: https://phabricator.kde.org/D16504