media: dvb frontend docs: use kernel-doc documentation

Now that frontend.h contains most documentation for the frontend,
remove the duplicated information from Documentation/ and use the
kernel-doc auto-generated one instead.

That should simplify maintainership of DVB frontend uAPI, as most
of the documentation will stick with the header file.

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