drm/doc: Polish plane composition property docs

Try to spec a bit more precisely how they all fit together, now that
at least the code is for all the additional properties is in one
place.

Also remove the entries for the standardized properties from the
table, because that thing is supremely unmaintaineable.

v2: Fix typos Sean spotted.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1474448370-32227-4-git-send-email-daniel.vetter@ffwll.ch