Added the following text at the end of 7.2 (which is now the introduction to the
entire NAT traversal discussion):

   In all cases, GIST interaction with the NAT is determined by the way
   the NAT handles the Query/Response messages in the initial GIST
   handshake; these messages are UDP datagrams.  Best current practice
   for NAT treatment of UDP traffic is defined in [28], and the legacy
   NAT handling defined in this specification is fully consistent with
   that document.  The GIST-aware NAT traversal technique is equivalent
   to requiring an Application Layer Gateway in the NAT for a specific
   class of UDP transactions, namely those where the destination UDP
   port for the initial message is the GIST port (see Section 9).