Please clarify different roles of Future from std vs library (and FuturesExt)

[tekul]

When reading the "under the hood" chapter I was confused because the definition of the "real" Future didn't include methods like boxed which the chapter uses later. It was only after some time that I realised these were defined in FuturesExt. At this point I wasn't aware of the other book at books.async.rs, but I now see that it makes these relationships clear early in the introduction to avoid confusion.

~~Perhaps it would be an idea to include a link to the async-std docs at the start of this book?~~ (Edit: I now see that the other book/async-std is not actually part of standard rust but also a separate crate :thinking: ) . I think a similar explanation would be very useful in this book, as well as some mention of FuturesExt as the source of methods like boxed.