media: dvb uapi: move frontend legacy API to another part of the book

There's a chapter for the legacy APIs. Move the frontend DVBv3
API to it, and update the chapter's introduction accordingly.

Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

Documentation/media/uapi/dvb/frontend.rst

···
     dvb-fe-read-status
     dvbproperty
     frontend_fcalls
-    frontend_legacy_dvbv3_api

Documentation/media/uapi/dvb/legacy_dvb_apis.rst

···
 Digital TV Deprecated APIs
 ***************************
 
-The APIs described here are kept only for historical reasons. There's
-just one driver for a very legacy hardware that uses this API. No modern
-drivers should use it. Instead, audio and video should be using the V4L2
-and ALSA APIs, and the pipelines should be set using the Media
-Controller API
+The APIs described here **should not** be used on new drivers or applications.
 
-.. note::
+The DVBv3 frontend API has issues with new delivery systems, including
+DVB-S2, DVB-T2, ISDB, etc.
+
+There's just one driver for a very legacy hardware using the Digital TV
+audio and video APIs. No modern drivers should use it. Instead, audio and
+video should be using the V4L2 and ALSA APIs, and the pipelines should
+be set via the Media Controller API.
+
+.. attention::
 
    The APIs described here doesn't necessarily reflect the current
-   code implementation.
+   code implementation, as this section of the document was written
+   for DVB version 1, while the code reflects DVB version 3
+   implementation.
 
 
 .. toctree::
     :maxdepth: 1
 
+    frontend_legacy_dvbv3_api
     video
     audio