[Update vector_env.py]: add some documentations (openai#1629)

* Update vector_env.py

* Update vector_env.py

gym/vector/vector_env.py

@@ -6,7 +6,16 @@
 
 
 class VectorEnv(gym.Env):
-    """Base class for vectorized environments.
+    r"""Base class for vectorized environments.
+
+    Each observation returned from vectorized environment is a batch of observations 
+    for each sub-environment. And :meth:`step` is also expected to receive a batch of 
+    actions for each sub-environment.
+    
+    .. note::
+    
+        All sub-environments should share the identical observation and action spaces.
+        In other words, a vector of multiple different environments is not supported. 
 
     Parameters
     ----------
@@ -40,7 +49,8 @@ def reset_wait(self, **kwargs):
         raise NotImplementedError()
 
     def reset(self):
-        """
+        r"""Reset all sub-environments and return a batch of initial observations.
+        
         Returns
         -------
         observations : sample from `observation_space`
@@ -56,7 +66,8 @@ def step_wait(self, **kwargs):
         raise NotImplementedError()
 
     def step(self, actions):
-        """
+        r"""Take an action for each sub-environments. 
+
         Parameters
         ----------
         actions : iterable of samples from `action_space`
@@ -74,8 +85,9 @@ def step(self, actions):
             A vector whose entries indicate whether the episode has ended.
 
         infos : list of dict
-            A list of auxiliary diagnostic informations.
+            A list of auxiliary diagnostic information dicts from sub-environments.
         """
+
         self.step_async(actions)
         return self.step_wait()