[media] tuner.h: rename TUNER_PAD_IF_OUTPUT to TUNER_PAD_OUTPUT

The output of a tuner is not only IF frequencies. They may also
output audio on some of its pins, and may even be a zero-IF tuner,
with outputs a baseband. So, rename the PAD name to make it
clearer and add a proper documentation about that at tuner.h.

No functional changes.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>