ALSA: compress: document the compress audio state machine

So we had some discussions of the stream states, so I thought it is a
good idea to document the state transitions, so add it documentation

Reviewed-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>
Link: https://lore.kernel.org/r/20200629134737.105993-2-vkoul@kernel.org


Signed-off-by: Takashi Iwai <tiwai@suse.de>