Documentation Frozen Lake : precise is_slippery true by default (#136)

Co-authored-by: Mark Towers <mark.m.towers@gmail.com>

gymnasium/envs/toy_text/frozen_lake.py

@@ -142,7 +142,9 @@ class FrozenLakeEnv(Env):
             "FFFHFFFG",
         ]
 
-    `is_slippery`: True/False. If True will move in intended direction with
+
+    #### Option is_slippery
+    Boolean, set true by default. When True, will move in intended direction with
     probability of 1/3 else will move in either perpendicular direction with
     equal probability of 1/3 in both directions.
 
@@ -151,6 +153,12 @@ class FrozenLakeEnv(Env):
         - P(move up)=1/3
         - P(move down)=1/3
 
+    To init the environment without a default value, specify explicitly the value of is_slippery
+    in the make command:
+
+        import gymnasium as gym
+        gym.make('FrozenLake-v1', desc=None, map_name="4x4", is_slippery=True)
+
     ## Version History
     * v1: Bug fixes to rewards
     * v0: Initial versions release (1.0.0)