Mention that BTreeMap::new() doesn't allocate

I think it would be nice to mention this, so you don't have to dig through the src to look at the definition of new().
rust-lang · Manishearth · May 30, 2020 · May 30, 2020 · Jun 1, 2020 · Jun 18, 2020
commit 0c5c644c91edf6ed949cfa5ffc524f43369df604
diff --git a/src/liballoc/collections/btree/map.rs b/src/liballoc/collections/btree/map.rs
@@ -490,6 +490,8 @@ struct MergeIter<K, V, I: Iterator<Item = (K, V)>> {
 impl<K: Ord, V> BTreeMap<K, V> {
     /// Makes a new empty BTreeMap with a reasonable choice for B.
     ///
+    /// Does not allocate anything on its own.
+    ///
     /// # Examples
     ///
     /// Basic usage: