Document that people no longer need iterator/asyncIterator shims (except IE11 etc)

[rattrayalex]

Async iterables are part of JS now, and available in all evergreen browsers and all non-EOL versions of Node.

Refs:

• https://caniuse.com/mdn-javascript_builtins_asynciterator_--asynciterator
• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol/asyncIterator