Remove duplicated term in glossary (#5379)

* remove duplicated Policy in glossary

* sort alphabetically

docs/Glossary.md

 - **Agent** - Unity Component which produces observations and takes actions in
   the environment. Agents actions are determined by decisions produced by a
   Policy.
-- **Policy** - The decision making mechanism, typically a neural network model.
+- **Experience** - Corresponds to a tuple of [Agent observations, actions,
+  rewards] of a single Agent obtained after a Step.
+- **External Coordinator** - ML-Agents class responsible for communication with
+  outside processes (in this case, the Python API).
 - **FixedUpdate** - Unity method called each time the game engine is stepped.
   ML-Agents logic should be placed here.
 - **Frame** - An instance of rendering the main camera for the display.
-- **Policy** - Function for producing decisions from observations.
+- **Policy** - The decision making mechanism for producing decisions from
+  observations, typically a neural network model.
 - **Reward** - Signal provided at every step used to indicate desirability of an
   agent’s action within the current state of the environment.
 - **State** - The underlying properties of the environment (including all agents
-- **Experience** - Corresponds to a tuple of [Agent observations, actions,
-  rewards] of a single Agent obtained after a Step.
-- **Update** - Unity function called each time a frame is rendered. ML-Agents
-  logic should not be placed here.
-- **External Coordinator** - ML-Agents class responsible for communication with
-  outside processes (in this case, the Python API).
+- **Update** - Unity function called each time a frame is rendered. ML-Agents
+  logic should not be placed here.