Document which steps in onion service connection can be parallelized

See points of discussion 3 through 6 at arti#913 (closed). For each point, we should document whether parallelizing these operations is a SHOULD, SHOULD NOT, MUST, MUST NOT, or MAY.

Point 3: Requesting a HsDesc from multiple HsDirs in parallel.

Point 4: Connecting to multiple introduction points and sending them INTRODUCE1 messages in parallel.

Point 5: Building rendezvous circuits and introduction circuits in parallel.

Point 6: Sending ESTABLISH_RENDEZVOUS and INTRODUCE1 messages in parallel.

I think that the outcome of these discussions was "none of these is a MUST". I'll try to narrow even more.

(If any of these don't make sense, have a look at arti#913 (closed) where @Diziet and @dgoulet express them in better detail.)