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.
Typo: humn. 

However, I'd characterize the top-level differences as follows:

name: String guaranteed to be unique within a given scope at a particular time, used in resource URLs; encouraged to be provided by clients at creation time and to be human friendly; intended to facilitate creation idempotence, enforcement of space-uniqueness of singleton objects, distinguishing distinct entities, and referencing of particular entities across multiple operations

uid: Opaque, system-generated identifier guaranteed to be unique in time and space; intended to distinguish between historical occurrences of similar entities.