QMP/input-send-event: update document of union InputEvent

Signed-off-by: NAmos Kong <akong@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

qapi-schema.json

@@ -3231,6 +3231,11 @@
 #
 # Input event union.
 #
+# @key: Input event of Keyboard
+# @btn: Input event of pointer buttons
+# @rel: Input event of relative pointer motion
+# @abs: Input event of absolute pointer motion
+#
 # Since: 2.0
 ##
 { 'union' : 'InputEvent',