Document that __name__ is needed to define classes.

Author: Michael Howitz

CHANGES.rst

@@ -4,7 +4,7 @@ Changes
 5.2 (unreleased)
 ----------------
 
-- Nothing changed yet.
+- Document that ``__name__`` is needed to define classes.
 
 
 5.1 (2020-10-07)

docs/usage/basic_usage.rst

@@ -101,12 +101,15 @@ Necessary setup
 properly.
 
 To use classes in Python 3
-    ``__metaclass__`` must be set. Set it to ``type`` to use no custom metaclass.
+    * ``__metaclass__`` must be set. Set it to ``type`` to use no custom metaclass.
+    * ``__name__`` must be set. As classes need a namespace to be defined in.
+      It is the name of the module the class is defined in. You might set it to
+      an arbitrary string.
 
 To use ``for`` statements and comprehensions:
-    * ``_getiter_`` must point to an ``iter`` implementation. As an unguarded variant you might use 
+    * ``_getiter_`` must point to an ``iter`` implementation. As an unguarded variant you might use
       :func:`RestrictedPython.Eval.default_guarded_getiter`.
-       
+
     *  ``_iter_unpack_sequence_`` must point to :func:`RestrictedPython.Guards.guarded_iter_unpack_sequence`.
 
 To use ``getattr``