Copy pt STATUS method descriptions to the right place, clarify TRANSPORT.

This branch adds a note about the status of TRANSPORT in PT status messages: it is required if you want to work with older versions of Tor or Arti, but not for current versions of Arti or Tor.

Also, this branch merges the changes of !63 (merged) into the markdown pt-spec proper, so they will get rendered. These were accidentally merged into attic/text_formats, where nobody will see them. I have confirmed that nothing else has been merged into attic since the conversion. (Next I'm going to go through all the other open torspec merge requests, to make sure that they are not about to get caught by the same issue.)

Closes #267 (closed).