Update softvol

Hi Mark,

sorry for the delay, at last I forked and updated the softvol documentation, as you suggested 2 months ago in a private mail (18/05/2014).

Just for the memory, the related discussion for this change is available at:
https://www.mail-archive.com/alsa-user@lists.sourceforge.net/msg30566.html
Sorry that I forgot to include that in the commit message (or directly in the markdown page, I don't know where is the best place).

All the best,
elboulangero