[ALSA] some docs for the new emu10k1 multichannel functionality

Documentation
Not complete, or even spell checked, but in case I don't get a chance to
work on it again before 1.0.9, here is what i have so far.  It should go
in alsa-kernel/Documentation/emu10k1-jack.txt.

Signed-off-by: Lee Revell <rlrevell@joe-job.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>