docs: add forgotten features to 18.2.0 release notes

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewied-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: 18.2: <mesa-stable@lists.freedesktop.org>
(cherry picked from commit 743e11c1)