ALSA: hda - Add documentation for codec specific mixer controls of Analog codecs

ALSA: hda - Add documentation for codec specific mixer controls of Analog codecs

* Channel Mode
  This is an enum control to change the surround-channel setup,
  appears only when the surround channels are available.
  It gives the number of channels to be used, "2ch", "4ch" abd "6ch".
  According to the configuration, this also controls the
  jack-retasking of multi-I/O jacks.

* Independent HP
  When this enum control is enabled, the headphone output is routed
  from an individual stream (the third PCM such as hw:0,2) instead of
  the primary stream.

Signed-off-by: Raymond Yau <superquad.vortex2@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>