sysadmin-guide: Improve extension lock down docs
gnome-shell has had a disabled-extensions
key for a while, that
overrides extensions even if they are enabled via settings or session
mode.
When locking down the set of enabled extensions, the intention is most likely to not allow disabling them either, so document locking down the additional key too.
See gnome-shell#6045 (comment 1589449) for context.