Document the new Transfer Function control (and fix the documentation for
the other colorspace controls which were not quite correct).

Mention the support for 4:2:0 and more multiplanar formats.

Update the TODO list at the end.
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>