improve documentation for `Fn*` traits

I can think of a few things we may want to accomplish with the documentation of the `Fn`, `FnMut`, and `FnOnce` traits:
- the relationship between these traits and the closures that implement them
- examples of non-closure implementations
- the relationship between these traits and Rust's ownership semantics

add module-level documentation for `Fn*` traits

Describe how `Fn*` traits, closure types, and ownership semantics are
linked, and provide examples of higher-level functions that take `Fn*`s.

more examples for `Fn*` traits

create correct (though not yet elegant) examples for `FnMut` and `FnOnce`

add trait links to module-level documentation

third time's a charm!

argument -> capture for trait documentation

This wording will need to be supported with better examples for
capturing eventually.

correct `FnOnce` example

I also fixed some of the trait wording here to make the concept of
capturing clearer; though that still needs more work.

replace `x + x` with `x * 2` for `fn double`

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