Padding with invalid packets is OK

Closes #4738.

draft-ietf-quic-transport.md

@@ -4215,9 +4215,10 @@ meet size constraints; the endpoint MAY however discard such datagrams.
 A client MUST expand the payload of all UDP datagrams carrying Initial packets
 to at least the smallest allowed maximum datagram size of 1200 bytes by adding
 PADDING frames to the Initial packet or by coalescing the Initial packet; see
-{{packet-coalesce}}.  Similarly, a server MUST expand the payload of all UDP
-datagrams carrying ack-eliciting Initial packets to at least the smallest
-allowed maximum datagram size of 1200 bytes.
+{{packet-coalesce}}.  Initial packets can even be coalesced with invalid
+packets, which an receiver has to discard.  Similarly, a server MUST expand the
+payload of all UDP datagrams carrying ack-eliciting Initial packets to at least
+the smallest allowed maximum datagram size of 1200 bytes.
 
 Sending UDP datagrams of this size ensures that the network path supports a
 reasonable Path Maximum Transmission Unit (PMTU), in both directions.