qmp: qmp-events.txt: add missing doc for the SUSPEND event

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>

QMP/qmp-events.txt

@@ -240,6 +240,18 @@ Example:
 { "event": "STOP",
     "timestamp": { "seconds": 1267041730, "microseconds": 281295 } }
 
+SUSPEND
+-------
+
+Emitted when guest enters S3 state.
+
+Data: None.
+
+Example:
+
+{ "event": "SUSPEND",
+     "timestamp": { "seconds": 1344456160, "microseconds": 309119 } }
+
 VNC_CONNECTED
 -------------