media: frontend.rst: mention MMT at the documentation

The ATSC 3.0 uses MPEG Media Transport, with is not currently
supported. Yet, we'll need to implement it sooner or later.
So, mention about it at the specs.

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

Documentation/media/uapi/dvb/frontend.rst

@@ -33,9 +33,8 @@ Data types and ioctl definitions can be accessed by including
 
 .. note::
 
-   Transmission via the internet (DVB-IP) is not yet handled by this
-   API but a future extension is possible.
-
+   Transmission via the internet (DVB-IP) and MMT (MPEG Media Transport)
+   is not yet handled by this API but a future extension is possible.
 
 .. [#f1]