Remove some cruft from packet size section

This section had a bunch of text that wasn't updated as the result of
other changes elsewhere.  Though there was a suggestion that this could
reference other sections, but as this is in some ways the authoritative
section on the subject, I decided to keep the requirements.  That's
duplicative, but I couldn't see a simple way to deal with that.

Closes #3255.

draft-ietf-quic-transport.md

@@ -3410,27 +3410,22 @@ later time in the connection.
 The QUIC packet size includes the QUIC header and protected payload, but not the
 UDP or IP header.
 
-Clients MUST ensure they send the first Initial packet in a single IP packet.
-Similarly, the first Initial packet sent after receiving a Retry packet MUST be
-sent in a single IP packet.
-
-The payload of a UDP datagram carrying the first Initial packet MUST be expanded
-to at least 1200 bytes, by adding PADDING frames to the Initial packet and/or by
-coalescing the Initial packet (see {{packet-coalesce}}). Sending a UDP datagram
-of this size ensures that the network path supports a reasonable Maximum
-Transmission Unit (MTU), and helps reduce the amplitude of amplification attacks
-caused by server responses toward an unverified client address; see
+The payload of a UDP datagram carrying Initial packets from a client MUST be
+expanded to at least 1200 bytes, by adding PADDING frames to the Initial packet
+and/or by coalescing the Initial packet (see {{packet-coalesce}}). Sending a UDP
+datagram of this size ensures that the network path supports a reasonable
+Maximum Transmission Unit (MTU), and helps reduce the amplitude of amplification
+attacks caused by server responses toward an unverified client address; see
 {{address-validation}}.
 
-The datagram containing the first Initial packet from a client MAY exceed 1200
-bytes if the client believes that the Path Maximum Transmission Unit (PMTU)
-supports the size that it chooses.
+These datagrams containing Initial packets MAY exceed 1200 bytes if the client
+believes that the Path Maximum Transmission Unit (PMTU) supports the size that
+it chooses.
 
 A server MAY send a CONNECTION_CLOSE frame with error code PROTOCOL_VIOLATION in
-response to the first Initial packet it receives from a client if the UDP
-datagram is smaller than 1200 bytes. It MUST NOT send any other frame type in
-response, or otherwise behave as if any part of the offending packet was
-processed as valid.
+response to an Initial packet it receives from a client if the UDP datagram is
+smaller than 1200 bytes. It MUST NOT send any other frame type in response, or
+otherwise behave as if any part of the offending packet was processed as valid.
 
 The server MUST also limit the number of bytes it sends before validating the
 address of the client; see {{address-validation}}.