[docs] document Generator properties

bob/learn/tensorflow/data/generator.py

@@ -76,10 +76,12 @@ class Generator:
 
     @property
     def output_types(self):
+        "The types of the returned samples."
         return self._output_types
 
     @property
     def output_shapes(self):
+        "The shapes of the returned samples."
         return self._output_shapes
 
     def __call__(self):