Document both importlib.machinery.BuiltinImporter and FrozenImporter.
diff --git a/Lib/importlib/_bootstrap.py b/Lib/importlib/_bootstrap.py
index d11513a..c1d09dd 100644
--- a/Lib/importlib/_bootstrap.py
+++ b/Lib/importlib/_bootstrap.py
@@ -110,8 +110,8 @@
             return None
         return cls if imp.is_builtin(fullname) else None
 
-    @staticmethod
-    def load_module(fullname):
+    @classmethod
+    def load_module(cls, fullname):
         """Load a built-in module."""
         if fullname not in sys.builtin_module_names:
             raise ImportError("{0} is not a built-in module".format(fullname))