Clarify how connection ID can be omitted

The definition of the truncate_connection_id was correct, but the name
was poor (it implied that part of the connection ID was included) and it
wasn't referenced from the critical part of the document (the short
header definition).  This corrects both of those oversights.

Closes #659.

draft-ietf-quic-transport.md

@@ -468,7 +468,10 @@ Connection ID Flag:
 
 : The second bit (0x40) of the first octet indicates whether the Connection ID
   field is present.  If set to 1, then the Connection ID field is present; if
-  set to 0, the Connection ID field is omitted.
+  set to 0, the Connection ID field is omitted.  The Connection ID field can
+  only be omitted if the omit_connection_id transport parameter
+  ({{transport-parameter-definitions}}) is used by the intended recipient of the
+  packet.
 
 Key Phase Bit:
 
@@ -1045,7 +1048,7 @@ language from Section 3 of {{!I-D.ietf-tls-tls13}}.
       initial_max_data(1),
       initial_max_stream_id(2),
       idle_timeout(3),
-      truncate_connection_id(4),
+      omit_connection_id(4),
       max_packet_size(5),
       (65535)
    } TransportParameterId;
@@ -1123,12 +1126,13 @@ idle_timeout (0x0003):
 
 An endpoint MAY use the following transport parameters:
 
-truncate_connection_id (0x0004):
+omit_connection_id (0x0004):
 
-: The truncated connection identifier parameter indicates that packets sent to
-  the peer can omit the connection ID.  This can be used by an endpoint where
-  the 5-tuple is sufficient to identify a connection.  This parameter is zero
-  length.  Omitting the parameter indicates that the endpoint relies on the
+: The omit connection identifier parameter indicates that packets sent to the
+  endpoint that advertises this parameter can omit the connection ID.  This can
+  be used by an endpoint where it knows that source and destination IP address
+  and port are sufficient for it to identify a connection.  This parameter is
+  zero length.  Absence this parameter indicates that the endpoint relies on the
   connection ID being present in every packet.
 
 max_packet_size (0x0005):
@@ -3057,7 +3061,7 @@ The initial contents of this registry are shown in
 | 0x0001 | initial_max_data        | {{transport-parameter-definitions}} |
 | 0x0002 | initial_max_stream_id   | {{transport-parameter-definitions}} |
 | 0x0003 | idle_timeout            | {{transport-parameter-definitions}} |
-| 0x0004 | truncate_connection_id  | {{transport-parameter-definitions}} |
+| 0x0004 | omit_connection_id      | {{transport-parameter-definitions}} |
 | 0x0005 | max_packet_size         | {{transport-parameter-definitions}} |
 {: #iana-tp-table title="Initial QUIC Transport Parameters Entries"}