Docs: Update docs to reflect normalized UUID

[jrconlin]

All UUIDs are normalized to lower case with '-' separators. We need to note that in the docs so that app creators who use the UUID as a string index don't get random failures or unexpected results.

[ghost]

Let's also have process_register return an error for channel IDs that aren't in that form. I think this would be less surprising than our current behavior, where we accept the ID, but then deliver to the normalized form.

At first glance, I think all we need to do is replace the check here: https://github.com/mozilla-services/autopush/blob/master/autopush/websocket.py#L996-L997 with one that checks data["channelID"] == normalize_id(data["channelID"]).