diff --git a/collections/btree/map.rs b/collections/btree/map.rs
index dfd693d..79042ab 100644
--- a/collections/btree/map.rs
+++ b/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
     ///
diff --git a/collections/btree/set.rs b/collections/btree/set.rs
index be740b0..9711214 100644
--- a/collections/btree/set.rs
+++ b/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
     ///