Editing the GridSensor documentation for 2D use case (#5396)

* Editing the GridSensor documentation for 2D use case

* changing chagelog

com.unity.ml-agents/CHANGELOG.md

 ### Bug Fixes
 - The calculation of the target entropy of SAC with continuous actions was incorrect and has been fixed. (#5372)
 - RigidBodySensorComponent now displays a warning if it's used in a way that won't generate useful observations. (#5387)
+- Update the documentation with a note saying that `GridSensor` does not work in 2D environments. (#5396)
 
 
 ## [2.0.0-exp.1] - 2021-04-22

docs/Learning-Environment-Design-Agents.md

 [extending grid Sensors](https://github.com/Unity-Technologies/ml-agents/blob/release_17/com.unity.ml-agents.extensions/Documentation~/CustomGridSensors.md)
 for more details on custom grid sensors.
 
+__Note__: The `GridSensor` only works in 3D environments and will not behave
+properly in 2D environments.
+
 #### Grid Observation Summary & Best Practices
 
 - Attach `GridSensorComponent` to use.
   learning stability and agent performance.
+- Do not use `GridSensor` in a 2D game.
 
 ### Variable Length Observations