mixer: add documentation about postcondition of removal event processing

[GitHub pull_request - opened]

alsa-project/alsa-lib pull request #244 was opened from takaswie:

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.

Signed-off-by: Takashi Sakamoto <o-takashi at sakamocchi.jp>

Request URL   : https://github.com/alsa-project/alsa-lib/pull/244
Patch URL     : https://github.com/alsa-project/alsa-lib/pull/244.patch
Repository URL: https://github.com/alsa-project/alsa-lib