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.
Other components may append to some string in order to produce unique Names for objects they create, such as pods created by a replicationController from a podTemplate. (The system isn't going to munge a client-provided Name and thereby change the Name of an object directly created via the API.)