vnc: update docs/multiseat.txt

vnc joins the party ;)
Also some s/head/seat/ to clarify.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

docs/multiseat.txt

@@ -7,7 +7,7 @@ host side
 
 First you must compile qemu with a user interface supporting
 multihead/multiseat and input event routing.  Right now this
-list includes sdl2 and gtk (both 2+3):
+list includes sdl2, gtk (both 2+3) and vnc:
 
   ./configure --enable-sdl --with-sdlabi=2.0
 
@@ -16,16 +16,16 @@ or
   ./configure --enable-gtk
 
 
-Next put together the qemu command line:
+Next put together the qemu command line (sdk/gtk):
 
 qemu	-enable-kvm -usb $memory $disk $whatever \
 	-display [ sdl | gtk ] \
 	-vga std \
 	-device usb-tablet
 
-That is it for the first head, which will use the standard vga, the
+That is it for the first seat, which will use the standard vga, the
 standard ps/2 keyboard (implicitly there) and the usb-tablet.  Now the
-additional switches for the second head:
+additional switches for the second seat:
 
 	-device pci-bridge,addr=12.0,chassis_nr=2,id=head.2 \
 	-device secondary-vga,bus=head.2,addr=02.0,id=video.2 \
@@ -47,6 +47,16 @@ in a separate tab.  You can either simply switch tabs to switch heads,
 or use the "View / Detach tab" menu item to move one of the displays
 to its own window so you can see both display devices side-by-side.
 
+For vnc some additional configuration on the command line is needed.
+We'll create two vnc server instances, and bind the second one to the
+second seat, simliar to input devices:
+
+	-display vnc=:1,id=primary \
+	-display vnc=:2,id=secondary,display=video.2
+
+Connecting to vnc display :1 gives you access to the first seat, and
+likewise connecting to vnc display :2 shows the second seat.
+
 Note on spice: Spice handles multihead just fine.  But it can't do
 multiseat.  For tablet events the event source is sent to the spice
 agent.  But qemu can't figure it, so it can't do input routing.