[ALSA] Warning doc about VIA82xx recording

Documentation
Add warning about the consequences of adjusting the 'Input Source Select'
of VIA82xx.
Signed-off-by: Ross Kendall Axe <ross.axe@blueyonder.co.uk>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

Documentation/sound/alsa/VIA82xx-mixer.txt

+
+				VIA82xx mixer
+				=============
+
+On many VIA82xx boards, the 'Input Source Select' mixer control does not work.
+Setting it to 'Input2' on such boards will cause recording to hang, or fail
+with EIO (input/output error) via OSS emulation.  This control should be left
+at 'Input1' for such cards.