Merge pull request #2031 from quicwg/client-no-migrate

Servers can't migrate.  Period.

draft-ietf-quic-transport.md

@@ -1812,7 +1812,7 @@ This document limits migration of connections to new client addresses, except as
 described in {{preferred-address}}. Clients are responsible for initiating all
 migrations.  Servers do not send non-probing packets (see {{probing}}) toward a
 client address until they see a non-probing packet from that address.  If a
-client receives packets from an unknown server address, the client MAY discard
+client receives packets from an unknown server address, the client MUST discard
 these packets.