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.
Are locally created pods visible in the APIserver as API objects?  I'm guessing not at the moment.  In which case, how about just making this doc about APIserver object naming, and having a separate "local pods" doc about the special cases of node-locally created objects.