DOC document __builtins__ of modules are not pickled

cloudpickle/cloudpickle_fast.py

@@ -342,6 +342,10 @@ def _module_reduce(obj):
     if _is_importable(obj):
         return subimport, (obj.__name__,)
     else:
+        # Some external libraries can populate the "__builtins__" entry of a
+        # module's `__dict__` with unpicklable objects (see #316). For that
+        # reason, we do not attempt to pickle the "__builtins__" entry, and
+        # restore a default value for it at unpickling time.
         state = obj.__dict__.copy()
         state.pop('__builtins__', None)
         return dynamic_subimport, (obj.__name__, state)