V4L/DVB (12915): DocBook/media: Add isdb-t documentation

Adds ISDB-T_and_ISDB-Tsb_with_S2API spec converted into DocBook format.

The text is authored by Patrick Boettcher, and was converted to DocBook
by me.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
diff --git a/Documentation/DocBook/dvb/frontend.xml b/Documentation/DocBook/dvb/frontend.xml
index 028c9f2..9d89a7b 100644
--- a/Documentation/DocBook/dvb/frontend.xml
+++ b/Documentation/DocBook/dvb/frontend.xml
@@ -1763,3 +1763,4 @@
  </row></tbody></tgroup></informaltable>
 </section>
 </section>
+&sub-isdbt;