Update kcmsmserver docbook to 5.12
proofread + update
bump date + releaseinfo
replace kddm with sddm
replace kde with plasma
Differential Revision: https://phabricator.kde.org/D11817
(cherry picked from commit 4c627632a92cbea5a22176f3e9d3c39b9c6fb5f7)
backport to 5.12 LTS