Documentation for async

[benjamingr]

I think the documentation for .async is very misleading since it says:

An throttled version of Promise.all

A lot of people, in my experience answering questions in StackOverflow, being involved in the spec process and helping maintain a library is that a lot of people mix up the fact a promise is a proxy for the value of an already started operation.

A throttled Promise.all is impossible of course since all the inputs to .all are already promises which means the operations themselves have already started.

It would be useful to update the documentation to reflect that, also it would be nice if the docs were moved to an API.md file or something similar.