Revisit identifiers spec

Partially addresses #1135.

This is the result of discussions in the past few days about imprecision and confusion.  Hopefully simpler.  There are a few FIXME comments in the doc - discuss.
Predictable container names, as in someone could guess the container name and run a Docker command against it? Do we need/want that?

Otherwise, I think this applies to all API objects, not just pods.