[media] doc-rst: Fix format of avermedia.rst

This file is almost at the ReST format, but some things need
to be fixed for it to be parsed.

Also, the documentation there is old. So, add a notice about
that.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/media/dvb-drivers/index.rst

@@ -19,3 +19,4 @@ License".
 	:maxdepth: 5
 
 	intro
+	avermedia