media: docs: update avermedia.rst contents

While this is old, now that we moved the intro part of it,
its contents seem to be valid, if we mention that we're
talking only about the BT878 support.

Update the document title accordingly and remove the obsolete
note from it.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/admin-guide/media/avermedia.rst

 .. SPDX-License-Identifier: GPL-2.0
 
-HOWTO: Get An Avermedia DVB-T working under Linux
--------------------------------------------------
+======================================
+Avermedia DVB-T on BT878 Release Notes
+======================================
 
 February 14th 2006
 
 .. note::
 
-   This documentation is outdated. Please check at the DVB wiki
-   at https://linuxtv.org/wiki for more updated info.
+   Several other Avermedia devices are supported. For a more
+   broader and updated content about that, please check:
 
-   There's a section there specific for Avermedia boards at:
    https://linuxtv.org/wiki/index.php/AVerMedia
 
 The Avermedia DVB-T