Rollup merge of #72761 - poliorcetics:use-keyword-doc, r=Dylan-DPC

Added the documentation for the 'use' keyword

This is a partial fix of #34601.

I heavily inspired myself from the Reference on the `use` keyword.

I checked the links when compiling the documentation, they should be ok.

I also added an example for the wildcard `*` in the case of types, because it's behaviour is not *import everything* like one might think at first.

src/libstd/keyword_docs.rs

@@ -1213,9 +1213,61 @@ mod unsafe_keyword {}
 //
 /// Import or rename items from other crates or modules.
 ///
-/// The documentation for this keyword is [not yet complete]. Pull requests welcome!
+/// Usually a `use` keyword is used to shorten the path required to refer to a module item.
+/// The keyword may appear in modules, blocks and even functions, usually at the top.
+///
+/// The most basic usage of the keyword is `use path::to::item;`,
+/// though a number of convenient shortcuts are supported:
+///
+///   * Simultaneously binding a list of paths with a common prefix,
+///     using the glob-like brace syntax `use a::b::{c, d, e::f, g::h::i};`
+///   * Simultaneously binding a list of paths with a common prefix and their common parent module,
+///     using the [`self`] keyword, such as `use a::b::{self, c, d::e};`
+///   * Rebinding the target name as a new local name, using the syntax `use p::q::r as x;`.
+///     This can also be used with the last two features: `use a::b::{self as ab, c as abc}`.
+///   * Binding all paths matching a given prefix,
+///     using the asterisk wildcard syntax `use a::b::*;`.
+///   * Nesting groups of the previous features multiple times,
+///     such as `use a::b::{self as ab, c, d::{*, e::f}};`
+///   * Reexporting with visibility modifiers such as `pub use a::b;`
+///   * Importing with `_` to only import the methods of a trait without binding it to a name
+///     (to avoid conflict for example): `use ::std::io::Read as _;`.
+///
+/// Using path qualifiers like [`crate`], [`super`] or [`self`] is supported: `use crate::a::b;`.
+///
+/// Note that when the wildcard `*` is used on a type, it does not import its methods (though
+/// for `enum`s it imports the variants, as shown in the example below).
+///
+/// ```compile_fail,edition2018
+/// enum ExampleEnum {
+///     VariantA,
+///     VariantB,
+/// }
 ///
-/// [not yet complete]: https://github.com/rust-lang/rust/issues/34601
+/// impl ExampleEnum {
+///     fn new() -> Self {
+///         Self::VariantA
+///     }
+/// }
+///
+/// use ExampleEnum::*;
+///
+/// // Compiles.
+/// let _ = VariantA;
+///
+/// // Does not compile !
+/// let n = new();
+/// ```
+///
+/// For more information on `use` and paths in general, see the [Reference].
+///
+/// The differences about paths and the `use` keyword between the 2015 and 2018 editions
+/// can also be found in the [Reference].
+///
+/// [`crate`]: keyword.crate.html
+/// [`self`]: keyword.self.html
+/// [`super`]: keyword.super.html
+/// [Reference]: ../reference/items/use-declarations.html
 mod use_keyword {}
 
 #[doc(keyword = "where")]