qemu-options.hx: document -audiodev dbus

Document the backend introduced in commit 739362d420 ("audio: add dbus
audio backend").

Reported-by: Mark Cave-Ayland <mark.caveayland@nutanix.com>
Reviewed-by: Mark Cave-Ayland <mark.caveayland@nutanix.com>
Reviewed-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>

qemu-options.hx

@@ -991,6 +991,16 @@ SRST
     ``in|out.buffer-count=count``
         Sets the count of the buffers.
 
+``-audiodev dbus,id=id[,prop[=value][,...]]``
+    Creates a D-Bus backend. It must be associated with the display
+    (as ``-display dbus,audiodev=id``). (Since 7.0)
+
+    D-Bus specific options are:
+
+    ``nsamples``
+        Number of samples per read/write (default to 480, 10ms at 48kHz)
+        (Since 10.0)
+
 ``-audiodev dsound,id=id[,prop[=value][,...]]``
     Creates a backend using Microsoft's DirectSound. This backend is
     only available on Windows and only supports playback.