doc: add note regarding unfinished TLA

Now that the proposal has reached Stage 4, I think it's time to remove the link from our docs.

Trying to use await somewhere it's not allowed with a Chromium browser returns the following error: await is only valid in async functions and the top level bodies of modules. That's why I'm re-using the term "body" in this PR, please correct me if you think that's not correct.