add migration notes on Behavior Type (#3008)

Author: Chris Elion

docs/Migrating.md

@@ -5,6 +5,10 @@
 ### Important Changes
 * Text actions and observations, and custom action and observation protos have been removed.
 * RayPerception3D and RayPerception2D are marked deprecated, and will be removed in a future release. They can be replaced by RayPerceptionSensorComponent3D and RayPerceptionSensorComponent2D.
+* The `Use Heuristic` checkbox in Behavior Parameters has been replaced with a `Behavior Type` dropdown menu. This has the following options:
+  * `Default` corresponds to the previous unchecked behavior, meaning that Agents will train if they connect to a python trainer, otherwise they will performance inference.
+  * `Heuristic Only` means the Agent will always use the `Heuristic()` method. This corresponds to having "Use Heuristic" selected in 0.11.0.
+  * `Inference Only` means the Agent will always perform inference.
 
 ### Steps to Migrate
 * We [fixed a bug](https://github.com/Unity-Technologies/ml-agents/pull/2823) in `RayPerception3d.Perceive()` that was causing the `endOffset` to be used incorrectly. However this may produce different behavior from previous versions if you use a non-zero `startOffset`. To reproduce the old behavior, you should increase the the value of `endOffset` by `startOffset`. You can verify your raycasts are performing as expected in scene view using the debug rays.