Tighten the identifiers spec

As I read through it I wanted to wordsmith a bit, which lead to reordering
some to read sort of top-down. There were a couple things I thought were wrong
(pod name should be a subdomain) and a few things I think we can make more
concise. I also added examples and clarified implications.
Merge pull request #349 from thockin/cleanups

WIP: tighten identifier spec