Improve {BTreeMap,HashMap}::get_key_value docs.

They are unusual methods. The docs don't really describe the cases when
they might be useful (as opposed to just `get`), and the examples don't
demonstrate the interesting cases at all.

This commit improves the docs and the examples.

Author: nnethercote

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

@@ -705,20 +705,55 @@ impl<K, V, A: Allocator + Clone> BTreeMap<K, V, A> {
         }
     }
 
-    /// Returns the key-value pair corresponding to the supplied key.
+    /// Returns the key-value pair corresponding to the supplied key. This is
+    /// only useful for key types where non-identical keys can be considered equal.
     ///
     /// The supplied key may be any borrowed form of the map's key type, but the ordering
     /// on the borrowed form *must* match the ordering on the key type.
     ///
     /// # Examples
     ///
     /// ```
+    /// use std::cmp::Ordering;
     /// use std::collections::BTreeMap;
     ///
+    /// #[derive(Clone, Copy, Debug)]
+    /// struct S {
+    ///     id: u32,
+    /// #   #[allow(unused)] // prevents a "field `name` is never read" error
+    ///     name: &'static str, // ignored by equality and ordering operations
+    /// }
+    ///
+    /// impl PartialEq for S {
+    ///     fn eq(&self, other: &S) -> bool {
+    ///         self.id == other.id
+    ///     }
+    /// }
+    ///
+    /// impl Eq for S {}
+    ///
+    /// impl PartialOrd for S {
+    ///     fn partial_cmp(&self, other: &S) -> Option<Ordering> {
+    ///         self.id.partial_cmp(&other.id)
+    ///     }
+    /// }
+    ///
+    /// impl Ord for S {
+    ///     fn cmp(&self, other: &S) -> Ordering {
+    ///         self.id.cmp(&other.id)
+    ///     }
+    /// }
+    ///
+    /// let j_a = S { id: 1, name: "Jessica" };
+    /// let j_b = S { id: 1, name: "Jess" };
+    /// let p = S { id: 2, name: "Paul" };
+    /// assert_eq!(j_a, j_b);
+    ///
     /// let mut map = BTreeMap::new();
-    /// map.insert(1, "a");
-    /// assert_eq!(map.get_key_value(&1), Some((&1, &"a")));
-    /// assert_eq!(map.get_key_value(&2), None);
+    /// map.insert(j_a, "Paris");
+    /// assert_eq!(map.get_key_value(&j_a), Some((&j_a, &"Paris")));
+    /// assert_eq!(map.get_key_value(&j_b), Some((&j_a, &"Paris"))); // the notable case
+    /// assert_eq!(map.get_key_value(&p), None);
     /// ```
     #[stable(feature = "map_get_key_value", since = "1.40.0")]
     pub fn get_key_value<Q: ?Sized>(&self, k: &Q) -> Option<(&K, &V)>

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

@@ -880,7 +880,8 @@ where
         self.base.get(k)
     }
 
-    /// Returns the key-value pair corresponding to the supplied key.
+    /// Returns the key-value pair corresponding to the supplied key. This is
+    /// only useful for key types where non-identical keys can be considered equal.
     ///
     /// The supplied key may be any borrowed form of the map's key type, but
     /// [`Hash`] and [`Eq`] on the borrowed form *must* match those for
@@ -890,11 +891,39 @@ where
     ///
     /// ```
     /// use std::collections::HashMap;
+    /// use std::hash::{Hash, Hasher};
+    ///
+    /// #[derive(Clone, Copy, Debug)]
+    /// struct S {
+    ///     id: u32,
+    /// #   #[allow(unused)] // prevents a "field `name` is never read" error
+    ///     name: &'static str, // ignored by equality and hashing operations
+    /// }
+    ///
+    /// impl PartialEq for S {
+    ///     fn eq(&self, other: &S) -> bool {
+    ///         self.id == other.id
+    ///     }
+    /// }
+    ///
+    /// impl Eq for S {}
+    ///
+    /// impl Hash for S {
+    ///     fn hash<H: Hasher>(&self, state: &mut H) {
+    ///         self.id.hash(state);
+    ///     }
+    /// }
+    ///
+    /// let j_a = S { id: 1, name: "Jessica" };
+    /// let j_b = S { id: 1, name: "Jess" };
+    /// let p = S { id: 2, name: "Paul" };
+    /// assert_eq!(j_a, j_b);
     ///
     /// let mut map = HashMap::new();
-    /// map.insert(1, "a");
-    /// assert_eq!(map.get_key_value(&1), Some((&1, &"a")));
-    /// assert_eq!(map.get_key_value(&2), None);
+    /// map.insert(j_a, "Paris");
+    /// assert_eq!(map.get_key_value(&j_a), Some((&j_a, &"Paris")));
+    /// assert_eq!(map.get_key_value(&j_b), Some((&j_a, &"Paris"))); // the notable case
+    /// assert_eq!(map.get_key_value(&p), None);
     /// ```
     #[inline]
     #[stable(feature = "map_get_key_value", since = "1.40.0")]