add note about re-requesting megolm keys

proposals/1719-olm_unwedging.md

@@ -19,6 +19,12 @@ from; the client should not create a new session with another device if it has
 already created one for that given device in the past 1 hour. (TODO: is 1 hour
 the right amount of time?)
 
+Clients may wish to ask the sender of the undecryptable messages to re-send the
+message.  For exampe, if the undecryptable message was a megolm session, then
+the client can send an
+[`m.room_key_request`](https://matrix.org/docs/spec/client_server/r0.4.0.html#m-room-key-request)
+message to request that the sender re-send the key.
+
 The spec currently says, "If a client has multiple sessions established with
 another device, it should use the session from which it last received a
 message." (the last paragraph of the [`m.olm.v1.curve25519-aes-sha2`