Update kcmnotify docbook to 5.12

proofread + update
bump date + releaseinfo
add how to disable all sound notifications
replace kde with plasma
remove commented option Speech

Differential Revision: https://phabricator.kde.org/D11693

(cherry picked from commit 80db7f6151f66f4e039b74a610dab89dfcf2c7c0)

backport to 5.12 LTS