media: ca.h: document most CA data types

For most of the stuff there, documenting is easy, as the
header file contains information.

Yet, I was unable to document two data structs:
	ca_msg and ca_descr

As those two structs are used by a few drivers, keep them.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>