Added definition of "collection states" and noted that informer cache
queries are answered against these.
Also added an explicit note that the concept of object identity used
in the contract here does _not_ include ObjectMeta.UID.
Also updated the description of the notifications received by a
client, to account for the fact that a client can be added after the
informer starts running.
Added references to `MetaNamespaceKeyFunc` and
`SplitMetaNamespaceKey`.
Noted that a client must process each notification promptly and added
a referral to workqueue.
Made a couple other minor clarifications and corrections.
Kubernetes-commit: 848301e11a8558f6a90e2fada870a91baace812a
0c0e2ef3a2