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.
Do we still need this?

We should not call this "namespace", since that currently collides with the namespace proposal.

Also, in the case of replicated apiservers, they should all share the same namespace. So, this is really a type of administrative domain. I suggest punting this topic to #1114.