Document the 'three-hops' restriction for circuit-build-timeout measurements

circuit_timeout_want_to_count_circ implements some slightly subtle logic: it counts a circuit as appropriate for CBT measurement if and only if:

it hasn't yet opened
when it is complete, it is intended to be at least DEFAULT_ROUTE_LEN (3) hops long
it currently has no more than DEFAULT_ROUTE_LEN (3) hops open.

Or briefly: a circuit is counted towards CBT only when its third hop opens. Later hops don't count, and shorter circuits don't count.

I tried to find this written down in path-spec.txt, but didn't. We should figure out a way to phrase this so that it encompasses the possibilities of a different "default".

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