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.
It needs to be unique within a #1114 namespace; it is not clear this is the same as what was meant by apiserver's namespace.   I'd leave out all mention of namespaces, and then commit this, and then ask @derekwaynecarr to update this file as part of #1114.