[std][BTree] Update doc-comment

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

@@ -104,7 +104,7 @@ impl<K: Ord, V, I> Iterator for MergeIter<K, V, I>
 {
     type Item = (K, V);
 
-    /// If two keys are equal, returns the key-value pair from the right source.
+    /// If two keys are equal, returns the key from the left and the value from the right.
     fn next(&mut self) -> Option<(K, V)> {
         let (a_next, b_next) = self.0.nexts(|a: &(K, V), b: &(K, V)| K::cmp(&a.0, &b.0));
         match (a_next, b_next) {