Update dir-spec with client fallback directory mirror attempt and timeout behaviour

Let's add these lines:

Clients try several fallback directory mirrors, and use the first one that connects. Each attempt happens after a short delay, regardless of the state of the previous attempt, until at least one attempt has connected.

When several fallback directory mirrors have failed, clients start trying directory authorities in a similar fashion.

Somewhere near:

https://gitweb.torproject.org/torspec.git/tree/dir-spec.txt#n3292

I don't think we designed any explicit timeout behaviour, so we are probably using whatever was there before 0.2.8.