media: docs: uAPI: dvb/audio: completing the documentation (function calls)

The existing documentation of the legacy DVB Decoder API was incomplete.

Revising the documentation, adding missing parts and arranging the
documentation files new.

This patch contains the documentation of the function calls defined in
audio.h.

Link: https://lore.kernel.org/linux-media/20240128233249.32794-5-herdler@nurfuerspam.de


Signed-off-by: Stefan Herdler <herdler@nurfuerspam.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>