ALSA: compress: document the compress gapless audio state machine

Also documented the galpess transitions. Please note that these are not
really stream states, but show how the stream steps in gapless mode

Signed-off-by: Vinod Koul <vkoul@kernel.org>
Link: https://lore.kernel.org/r/20200629134737.105993-3-vkoul@kernel.org


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