Add more links between hash and btree collections

- Link from `core::hash` to `HashMap` and `HashSet`
- Link from HashMap and HashSet to the module-level documentation on
  when to use the collection
- Link from several collections to Wikipedia articles on the general
  concept

library/alloc/src/collections/btree/map.rs

@@ -20,15 +20,15 @@ use Entry::*;
 /// We might temporarily have fewer elements during methods.
 pub(super) const MIN_LEN: usize = node::MIN_LEN_AFTER_SPLIT;
 
-// A tree in a `BTreeMap` is a tree in the `node` module with addtional invariants:
+// A tree in a `BTreeMap` is a tree in the `node` module with additional invariants:
 // - Keys must appear in ascending order (according to the key's type).
 // - If the root node is internal, it must contain at least 1 element.
 // - Every non-root node contains at least MIN_LEN elements.
 //
-// An empty map may be represented both by the absense of a root node or by a
+// An empty map may be represented both by the absence of a root node or by a
 // root node that is an empty leaf.
 
-/// A map based on a B-Tree.
+/// A map based on a [B-Tree].
 ///
 /// B-Trees represent a fundamental compromise between cache-efficiency and actually minimizing
 /// the amount of work performed in a search. In theory, a binary search tree (BST) is the optimal
@@ -62,6 +62,7 @@ pub(super) const MIN_LEN: usize = node::MIN_LEN_AFTER_SPLIT;
 /// undefined behavior. This could include panics, incorrect results, aborts, memory leaks, and
 /// non-termination.
 ///
+/// [B-Tree]: https://en.wikipedia.org/wiki/B-tree
 /// [`Cell`]: core::cell::Cell
 /// [`RefCell`]: core::cell::RefCell
 ///

library/core/src/hash/mod.rs

@@ -1,7 +1,13 @@
 //! Generic hashing support.
 //!
-//! This module provides a generic way to compute the hash of a value. The
-//! simplest way to make a type hashable is to use `#[derive(Hash)]`:
+//! This module provides a generic way to compute the [hash] of a value.
+//! Hashes are most commonly used with [`HashMap`] and [`HashSet`].
+//!
+//! [hash]: https://en.wikipedia.org/wiki/Hash_function
+//! [`HashMap`]: ../../std/collections/struct.HashMap.html
+//! [`HashSet`]: ../../std/collections/struct.HashSet.html
+
+//! The simplest way to make a type hashable is to use `#[derive(Hash)]`:
 //!
 //! # Examples
 //!

library/std/src/collections/hash/map.rs

@@ -15,7 +15,7 @@ use crate::iter::{FromIterator, FusedIterator};
 use crate::ops::Index;
 use crate::sys;
 
-/// A hash map implemented with quadratic probing and SIMD lookup.
+/// A [hash map] implemented with quadratic probing and SIMD lookup.
 ///
 /// By default, `HashMap` uses a hashing algorithm selected to provide
 /// resistance against HashDoS attacks. The algorithm is randomly seeded, and a
@@ -60,6 +60,7 @@ use crate::sys;
 /// The original C++ version of SwissTable can be found [here], and this
 /// [CppCon talk] gives an overview of how the algorithm works.
 ///
+/// [hash map]: crate::collections#use-a-hashmap-when
 /// [hashing algorithms available on crates.io]: https://crates.io/keywords/hasher
 /// [SwissTable]: https://abseil.io/blog/20180927-swisstables
 /// [here]: https://github.com/abseil/abseil-cpp/blob/master/absl/container/internal/raw_hash_set.h

library/std/src/collections/hash/set.rs

@@ -19,7 +19,7 @@ use super::map::{map_try_reserve_error, RandomState};
 // for `bucket.val` in the case of HashSet. I suppose we would need HKT
 // to get rid of it properly.
 
-/// A hash set implemented as a `HashMap` where the value is `()`.
+/// A [hash set] implemented as a `HashMap` where the value is `()`.
 ///
 /// As with the [`HashMap`] type, a `HashSet` requires that the elements
 /// implement the [`Eq`] and [`Hash`] traits. This can frequently be achieved by
@@ -105,6 +105,7 @@ use super::map::{map_try_reserve_error, RandomState};
 /// // use the values stored in the set
 /// ```
 ///
+/// [hash set]: crate::collections#use-the-set-variant-of-any-of-these-maps-when
 /// [`HashMap`]: crate::collections::HashMap
 /// [`RefCell`]: crate::cell::RefCell
 /// [`Cell`]: crate::cell::Cell