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.
This should be clarified to be:

"A non-empty string guaranteed to be unique to a resource type within a given scope at a particular time."

It should be a-ok to have different resource types that have the same name in a particular scope.