[media] media-entity.h: document all the structs

Only a few structs are documented on kernel-doc-nano format
(the ones added by the MC next gen patches).

Add a documentation for all structs, and ensure that they'll
be producing the documentation at the Kernel's device driver
DocBook.

Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>