docs/multiseat.txt: add note about spice

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

docs/multiseat.txt

@@ -47,6 +47,14 @@ 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.
 
+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.
+Fixing this needs a new or extended input interface between
+libspice-server and qemu.  For keyboard events it is even worse:  The
+event source isn't included in the spice protocol, so the wire
+protocol must be extended to support this.
+
 
 guest side
 ----------