mixer: add documentation about postcondition of removal event processing

mixer: add documentation about postcondition of removal event processing

PulseAudio and PipeWire seems to appear including careless code to
process events of mixer element in implementation of mixer class.

 * https://lore.kernel.org/alsa-devel/YrbxZ2b+3rIdi7Ut@workstation/

They register own implementation of mixer class. At addition event,
they attach own mixer element into hcontrol element. However at removal
event, they never detach the mixer element from hcontrol element. They
hit assertion in mixer API internal due to unsatisfied postcondition.

This commit adds documentation about postcondition of removal event
processing so that developer for implementation of mixer class easily
realize the way to satisfy the postcondition.

Fixes: https://github.com/alsa-project/alsa-lib/pull/244
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>