Document relay commands for circpad cells.

  Because circuit padding machines only become active at certain points in

  circuit lifetime, and because more than one padding machine may be active at

  any given point in circuit lifetime, there is also a padding negotiation cell,

  with fields as follows:

  any given point in circuit lifetime, there is also a padding negotiation

  cell and a negotiated response. These are relay commands 41 and 42, with

  relay headers as per section 6.1 of tor-spec.txt.

  The fields of the relay cell Data payload of a negotiate request are

  as follows:

     const CIRCPAD_COMMAND_STOP = 1;

     const CIRCPAD_COMMAND_START = 2;

  When a relay receives a circpad_negotiate cell, it checks that it supports

  the requested machine, and sends a circpad_negotiated cell, which is formatted

  as follows:

  in the data payload of a relay cell with command number 42 (see tor-spec.txt

  section 6.1), as follows:

     struct circpad_negotiated {

       u8 version IN [0];

        32..40 -- Used for hidden services; see rend-spec-{v2,v3}.txt.

        41..42 -- Used for circuit padding; see Section 3 of padding-spec.txt.

   Commands labelled as "forward" must only be sent by the originator

   of the circuit. Commands labelled as "backward" must only be sent by

   other nodes in the circuit back to the originator. Commands marked