axfer: add an explanation about Timer-based scheduling model

axfer: add an explanation about Timer-based scheduling model

This commit fulfills a subsection titled as 'Timer-based scheduling model'.

This scheduling model is introduced in a recent decade. In this model,
applications should take care of its timing to operate sampled data
according to any timer. This is an optional behaviour of runtime of
PCM substream.

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