D11607: Update systemsettings docbook to 5.12

[Marco Martin]

mart accepted this revision.
This revision is now accepted and ready to land.

REPOSITORY
  R124 System Settings

REVISION DETAIL
  https://phabricator.kde.org/D11607

To: lueck, #plasma, #documentation, mart
Cc: plasma-devel, kde-doc-english, ZrenBot, lesliezhai, ali-mohamed, jensreuterberg, abetts, sebas, apol, mart
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