Adjust reference to unidiomatic syntax in std docs roots

[withoutboats]

The opening section of the std documentation says this:

//! `std` is available to all Rust crates by default, just as if each one
//! contained an `extern crate std;` import at the [crate root]. Therefore the
//! standard library can be accessed in [`use`] statements through the path
//! `std`, as in [`use std::env`], or in expressions through the absolute path
//! `::std`, as in [`::std::env::args`].

This information, while accurate in both editions, is unidiomatic in the 2018 edition, in that it refers to extern crate syntax and double colon absolute paths. Perhaps this information could be reworded to be expressed in a more neutral way.