Document ring_size / n_val / physical node combinations where resilience promises break down [JIRA: PMT-168]

[jaredmorrow]

See basho/riak#381 we should document these in the ring_size section of the docs. I searched around and didn't see any mention (though it has been mentioned a ton in the riak-users mailing list).

Closing the basho/riak issue in favor of this one.

[DSomogyi]

Comment for Jira.

[lrnrthr]

Fixed by: #2352