`yield*` is not well documented

[jamesderlin]

#510 added yield* to the language tour, but it just says:

If your generator is recursive, you can improve its performance by using yield*:

which is a bit vague and doesn't explain what yield* actually does.

The old Dart Language Asynchrony Support: Phase 2 article had a more thorough explanation, and I can't find any counterpart on the current Dart site.

[atsansone]

@MaryaBelanger : Can you add this to the language work you're reviewing?