Improved taxi documentation (#2777)

* Improved documentation to specify decode state space

* Improve taxi documentation

gym/envs/toy_text/taxi.py

@@ -66,6 +66,12 @@ class TaxiEnv(Env):
     successful episodes, when both the passenger and the taxi are at the destination.
     This gives a total of 404 reachable discrete states.
 
+    Each state space is represented by the tuple:
+    (taxi_row, taxi_col, passenger_location, destination)
+
+    An observation is an integer that encodes the corresponding state.
+    The state tuple can then be decoded with the "decode" method.
+
     Passenger locations:
     - 0: R(ed)
     - 1: G(reen)
@@ -84,9 +90,6 @@ class TaxiEnv(Env):
     - +20 delivering passenger.
     - -10  executing "pickup" and "drop-off" actions illegally.
 
-    state space is represented by:
-    (taxi_row, taxi_col, passenger_location, destination)
-
     ### Arguments
 
     ```