tor-spec: describe END cell format better.

   Upon receiving this cell, the exit node resolves the address as

   necessary, and opens a new TCP connection to the target port.  If the

   address cannot be resolved, or a connection can't be established, the

   exit node replies with a RELAY_END cell.  (See 6.4 below.)

   exit node replies with a RELAY_END cell.  (See 6.3 below.)

   Otherwise, the exit node replies with a RELAY_CONNECTED cell, whose

   payload is in one of the following formats:

      14 -- REASON_NOTDIRECTORY   (Client sent RELAY_BEGIN_DIR to a

                                   non-directory relay.)

   (With REASON_EXITPOLICY, the 4-byte IPv4 address or 16-byte IPv6 address

   forms the optional data, along with a 4-byte TTL; no other reason

   currently has extra data.)

   For most reasons, the format is:

      Reason                      [1 byte]

   For REASON_EXITPOLICY, the format is:

      Reason                      [1 byte]

      IPv4 or IPv6 address        [4 bytes or 16 bytes]

      TTL                         [4 bytes]

   OPs and ORs MUST accept reasons not on the above list, since future

   versions of Tor may provide more fine-grained reasons.