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>