Document the return value of IndexMapCore::push

Author: cuviper

src/map_core.rs

@@ -176,7 +176,8 @@ impl<K, V> IndexMapCore<K, V> {
         }
     }
 
-    /// Append a key-value pair, *without* checking whether it already exists.
+    /// Append a key-value pair, *without* checking whether it already exists,
+    /// and return the pair's new index.
     fn push(&mut self, hash: HashValue, key: K, value: V) -> usize {
         let i = self.entries.len();
         self.indices.insert(hash.get(), i, get_hash(&self.entries));