add comments to AlfEnvironment.render

alf/environments/alf_environment.py

@@ -260,8 +260,10 @@ def render(self, mode='rgb_array'):
         """Renders the environment.
 
         Args:
-            mode: One of ['rgb_array', 'human']. Renders to an numpy array, or brings
+            mode: One of ['rgb_array', 'human', 'online']. Renders to an numpy array, or brings
                 up a window where the environment can be visualized.
+                The ``online`` mode can be used to collect and return a list of frames 
+                (e.g., generated by ``render(self, mode='rgb_array')``) of a running environment.
 
         Returns:
             An ndarray of shape ``[width, height, 3]`` denoting an RGB image if mode is