Rollup merge of #86790 - janikrabe:retain-iter-order-doc, r=m-ou-se

Document iteration order of `retain` functions

For `HashSet` and `HashMap`, this simply copies the comment from
`BinaryHeap::retain`.

For `BTreeSet` and `BTreeMap`, this adds an additional guarantee that
wasn't previously documented. I think that because these data structures
are inherently ordered and other functions guarantee ordered iteration,
it makes sense to provide this guarantee for `retain` as well.

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

@@ -935,6 +935,7 @@ impl<K, V> BTreeMap<K, V> {
     /// Retains only the elements specified by the predicate.
     ///
     /// In other words, remove all pairs `(k, v)` such that `f(&k, &mut v)` returns `false`.
+    /// The elements are visited in ascending key order.
     ///
     /// # Examples
     ///

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

@@ -846,6 +846,7 @@ impl<T> BTreeSet<T> {
     /// Retains only the elements specified by the predicate.
     ///
     /// In other words, remove all elements `e` such that `f(&e)` returns `false`.
+    /// The elements are visited in ascending order.
     ///
     /// # Examples
     ///

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

@@ -934,6 +934,7 @@ where
     /// Retains only the elements specified by the predicate.
     ///
     /// In other words, remove all pairs `(k, v)` such that `f(&k, &mut v)` returns `false`.
+    /// The elements are visited in unsorted (and unspecified) order.
     ///
     /// # Examples
     ///

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

@@ -912,6 +912,7 @@ where
     /// Retains only the elements specified by the predicate.
     ///
     /// In other words, remove all elements `e` such that `f(&e)` returns `false`.
+    /// The elements are visited in unsorted (and unspecified) order.
     ///
     /// # Examples
     ///