MFH: r537582

x11-servers/xwayland-devel: document DRI3 quirk

Wayland clients (unlike server/compositor) are not supposed to require
special privileges. Something in drm-kmod fails to authorize access to
/dev/dri/* even if user is under "video" group.

Not a port option because Xwayland doesn't know how to drop priveleges.

Approved by: ports-secteam blanket (runtime fix, no content changes)