doc-rst: Rename the title of the Digital TV section

The Digital TV section is ackward for two reasons:

  1) it is the only one with everything in upper case;
  2) its name is associated with the European digital TV standard.

Rename the part name, and add a notice that it refers to what's
known as "DVB API".
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/linux_tv/media/dvb/dvbapi.rst

@@ -4,9 +4,12 @@
 
 .. _dvbapi:
 
-#############
-LINUX DVB API
-#############
+##############
+Digital TV API
+##############
+
+**NOTE:** This API is also known as **DVB API**, although it is generic
+enough to support all digital TV standards.
 
 **Version 5.10**