docs: remove sound API duplication

The sound API is documented on two different parts:
under Documentation/driver-api/sound.rst and under
Documentation/sound/kernel-api/alsa-driver-api.rst.

The alsa-driver-api.rst seems more complete, and APIs
are split per type. There's just one missing kernel-doc
markup there.

Add it and drop the duplicated one.

Reviewed-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>