Update std/lib.rs docs to reflect Rust 2018 usage

Author: Xaeroxe

src/libstd/lib.rs

@@ -7,11 +7,9 @@
 //! primitives](#primitives), [standard macros](#macros), [I/O] and
 //! [multithreading], among [many other things][other].
 //!
-//! `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
+//! `std` is available to all Rust crates by default. 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`].
+//! `std`, as in [`use std::env`].
 //!
 //! # How to read this documentation
 //!