Missing documentation for asyncfutures

[andreaferretti]

https://nim-lang.org/docs/asyncfutures.html

I would provide a PR, but I am not sure what is there to do - isn't the documentation for the stdlib auto-generated from the modules themselves?

[twetzel59]

Oops, I just opened a forum topic about this...

[dom96]

https://github.com/nim-lang/Nim/blob/devel/web/website.ini#L31 :)

[andreaferretti]

Thank you! #7445

For these simple changes should I just commit to devel next time or do you prefer a PR anyway?

[Yardanico]

@andreaferretti this issue is about asyncfutures, but in your PR you add asyncstreams, is that correct?

[andreaferretti]

@Yardanico D'oh, you're right - in fact I needed both. I have added a commit with asyncfutures as well :-)