axfer: add an explanation about IRQ-based scheduling model

axfer: add an explanation about IRQ-based scheduling model

This commit adds a section titled as 'SCHEDULING MODEL' and fulfill a
subsection titled as 'IRQ-based scheduling model'.

This scheduling model is for a typical applications to operate a kind of
data sampled against actual time. In this model, ALSA PCM core maintains
timing of the applications with notification of hardware by
blocking/waking up their processes. This is a default behaviour of
runtime of PCM substream.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>