Update autostart docbook to 5.12

proofread + update
bump date + releseinfo
fix wording Pre-KDE startup -> Before session startup + KDE -> Plasma

Differential Revision: https://phabricator.kde.org/D11687

(cherry picked from commit b24fcb3a63ea4a838a94667f5511a4482896a075)

backport to 5.12 LTS