Update Future description

[JohnTitor]

Fixes #103

[ghost]

I kind of feel it would be best to just delete that sentence :) wdyt, @skade?

[skade]

@stjepang I'm not attached to the sentence as is, but we need to inform people of some way what the different kind of futures are and how the libraries relate.

[JohnTitor]

Hmm, what I should do? Should improve the sentence or remove it?

[ghost]

Hmm, perhaps we should do another pass over the whole section? It seems to imply a few things that are potentially confusing or misleading:

1. It says Future is a type, but it really is a trait.
2. It talks about differences between futures::future::Future and std::future::Future, but they are the same trait.

[yoshuawuyts]

ping @skade

[joodicator]

While reading https://book.async.rs/overview/std-and-library-futures.html, which at time of writing currently still incorrectly states that std::future::Future and futures::future::Future are distinct, I was confused, and it took me about half an hour to establish what was actually going on. It would be good to have this changed, since, as I can attest, the misinformation is having real-world consequences. Aside from that, I appreciate the information about asynchronous Rust.

[joshtriplett]

#1024

[joshtriplett]

Fixed by #1024