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.
I guess I was just confused by the subtle wording. Suggest putting (via uids) and (via Names) after the existing text. As English verbs, "identify" and "name" are pretty much identical. It's hard to infer what functionality we are implying, such as whether we make it easy to lookup and/or operate on the objects by identifier (now called uid) or name.