audio: omitting audiodev= parameter is only deprecated

Unfortunately, changes introduced in af2041ed2d "audio: audiodev=
parameters no longer optional when -audiodev present" breaks backward
compatibility.  This patch changes the error into a deprecation warning.

Signed-off-by: Kővágó, Zoltán <DirtY.iCE.hu@gmail.com>
Message-id: 02d4328c33455742d01e0b62395013e95293c3ba.1566847960.git.DirtY.iCE.hu@gmail.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
diff --git a/qemu-deprecated.texi b/qemu-deprecated.texi
index 00a4b6f..9d74a1c 100644
--- a/qemu-deprecated.texi
+++ b/qemu-deprecated.texi
@@ -72,6 +72,13 @@
 the new format, the ``-audiodev-help'' option can be used to convert
 the current values of the environment variables to ``-audiodev'' options.
 
+@subsection Creating sound card devices and vnc without audiodev= property (since 4.2)
+
+When not using the deprecated legacy audio config, each sound card
+should specify an @code{audiodev=} property.  Additionally, when using
+vnc, you should specify an @code{audiodev=} propery if you plan to
+transmit audio through the VNC protocol.
+
 @subsection -mon ...,control=readline,pretty=on|off (since 4.1)
 
 The @code{pretty=on|off} switch has no effect for HMP monitors, but is