audio: -audiodev command line option: documentation

This patch adds documentation of an -audiodev command line option, that
deprecates the old QEMU_* environment variables for audio backend
configuration.  It's syntax is similar to existing options (-netdev,
-device, etc):

  -audiodev driver_name,property=value,...

Although now it's possible to specify multiple -audiodev options on
command line, multiple audio backends are not supported yet.

Signed-off-by: Kővágó, Zoltán <DirtY.iCE.hu@gmail.com>
Message-id: ca5e761e58dcfaf591cf46080af3548551b42bb2.1552083282.git.DirtY.iCE.hu@gmail.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>