Update path-specs to educate about family

I'm not sure what Component I should select.

As suggested here: https://lists.torproject.org/pipermail/tor-talk/2013-April/027872.html

based on my email on tor-talk: https://lists.torproject.org/pipermail/tor-talk/2013-April/027869.html

I suggest to update the path-specs with an explanation on how excluding family works.

The dir-specs hold a nice (from my POV) explanation and gives an example.

If two ORs list one another in their "family" entries, then OPs should treat them as a single OR for the purpose of path selection. For example, if node A's descriptor contains "family B", and node B's descriptor contains "family A", then node A and node B should never be used on the same circuit.

The reason I suggest this is because I believe it is to be expected in the path-specs, rather than in the dir-spec. I'm not suggesting to remove it from any other document this explanation is in.