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.
There's the larger namespaces proposal, which segments a cluster, and the general idea of namespacing.  This is the latter.  If we end up having the former, it can replace this.

I'd be happy to add it back now, but I removed it at your request, so I am confused :)