Add explanation of main to rustdoc docs

[steveklabnik]

Fixes #17554

[rust-highfive]

r? @huonw

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[alexcrichton]

The full logic is actually a bit more complicated than this, but I'm not sure how much in detail it should be spelled out. Specifically, the steps taken are:

1. Given a code block, if it does not contain fn main, it is wrapped in fn main() { <codeblock> }
2. Given that result, if it contains no extern crate directives but it also contains the name of the crate being tested, then extern crate <name> is injected at the top.
3. Some common allow attributes are added for doc examples at the top.

This result is then tested.

Specifically, the example here isn't quite right, as what's actually tested look more like:

fn main() {
    use std::rc:Rc;
    let five = Rc::new(5);
}

[steveklabnik]

@alexcrichton I added the full thing, with some copyedit and formatting changes. I'm fine with just repeating it verbatim, I don't think this is incredibly likely to change.

[eddyb]

rc:Rc should be rc::Rc.