Improve description of what must be Fast or Stable

For arti#1100 (closed), I need to know which circuits require Fast or Stable relays. But the information in path-spec about Stable is a little outdated, and the information about Fast is basically non-existent.

I have tried to reverse-engineer stuff in tor a bit, and here is what I came up with:

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1. Only pick a node as a possible guard if it is Stable and Fast, in addition to having the Guard flag.

2. A circuit must have only Stable nodes...

• If it's for an introduction or rendezvous circuit.
  • (Actually, it seems that we don't enforce this for rendezvous circuits from onion services to the RP if none of our virtual ports is long-lived. This seems like a mistake to me, since re-creating a rendezvous circuit is somewhat expensive.)
• If we're going to use it for a long-lived port.

Notably, the following do not need to be stable:

• Ciruits used only for traffic on non-longlived-ports
• One-hop directory circuits
• Circuits built to HsDirs for uploading and downloading
• Self-testing circuits
• ...

3. A circuit must have only Fast nodes, unless...

• (I can't find any circuits like this.)

@mikeperry, do you have any ideas here? Am I on the right track?