[std][BTree] Update ::append docs

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

@@ -1160,6 +1160,10 @@ pub fn retain<F>(&mut self, mut f: F)
     ///
     /// If a key from `other` is already present in `self`, the respective
     /// value from `self` will be overwritten with the respective value from `other`.
+    /// Similar to [`insert`], though, the key is not overwritten,
+    /// which matters for types that can be `==` without being identical.
+    ///
+    /// [`insert`]: BTreeMap::insert
     ///
     /// # Examples
     ///