Is there documentation on the $A functions that are part of the published API (i.e., safe to use)?
Thank you for clarifying your question Peter. I didn't pick up on that nuance from the way you phrased your question. You've asked an excellent question in distinguishing between what currently "works", yet isn't documented in the SF API, and whether it's safe to use it in Aura Apps. Will be interesting to see what answers you receive.
Answering my own question. The Lightning Components Developer's Guide has the following:
So you can go to https://na15.lightning.force.com/auradocs/reference.app for example and see something like the following:
Please use caution when exploring the JS API. There are a lot of things that are used by the framework, but which aren't really useful when developing components.
What shows up on this list will be filtered soon to only include those items we plan for customers to use. The rest may change or be removed so you want to be wary of using anything from this for now.