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'd put a FIXME here regarding this issue, and we should hash this out in #1114 . There are several possible solutions, such as host-specific namespaces or autosetName=true. Namespace per apiserver is definitely not right given other discussions about replicating and/or sharding the apiservers, as well as namespacing by user-oriented scopes rather than (or in addition to) system-oriented ones. We will effectively need to namespace by store/etcd cluster, since a shared index is the underlying mechanism for enforcing uniqueness. However, as discussed in #1184, we could shard the store by resource kind, in which case namespacing would be implicit in the path part of the URL.