DOC: mention JSON files in docstring of method dd._abc.BDD.load

Author: johnyf

dd/_abc.py

@@ -241,7 +241,7 @@ def dump(
     # revise this method to return roots as `list` or
     # named nodes as `dict`
     def load(self, filename, levels=True):
-        """Load nodes from pickle file `filename`.
+        """Load nodes from pickle or JSON file `filename`.
 
         If `levels is True`,
         then load variables at the same levels.