Using "future" with a little `f`.

[ehuss]

There are a number of places that talk about a "future" with a lowercase f, but what a "future" is is never defined. It can maybe be inferred that it is an expression or type that implements std::future::Future.

I think, maybe, the solution here is to just add a glossary entry that states that explicitly, and maybe linking to it from a few places where there is no context given.

Alternatively, the places where the little f future is written, it could be written as Future with a capital F and a link. However, for some reason that seems less ideal to me.