bpo-26184: import.rst: Improve versionchanged note (GH-277)

Mention that an ImportError is raised when exec_module() is defined, but
create_module() is not.

Author: Mariatta

Doc/reference/import.rst

@@ -464,8 +464,11 @@ import machinery will create the new module itself.
 
 .. versionchanged:: 3.5
    A :exc:`DeprecationWarning` is raised when ``exec_module()`` is defined but
-   ``create_module()`` is not. Starting in Python 3.6 it will be an error to not
-   define ``create_module()`` on a loader attached to a ModuleSpec.
+   ``create_module()`` is not.
+
+.. versionchanged:: 3.6
+   An :exc:`ImportError` is raised when ``exec_module()`` is defined but
+   ``create_module`` is not.
 
 Submodules
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