[media] ca-get-msg.rst: add a boilerplate for struct ca_msg

[media] ca-get-msg.rst: add a boilerplate for struct ca_msg

There are no descriptions at ca.h header for this struct.
Yet, as we want to get rid of the warnings, let's add a
boilerplate, with just the struct types and fields.

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