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.
"identifier" is even more generic than "name". I advocate "uid" or "uniqueid" instead. Intentionally not uuid, though we may choose to use uuids to implement uid. I prefer uid, not just due to brevity, but also to avoid questions about what kind of uniqueness, why name isn't uniqueName, etc.