Migration guide for action/observation changes. (#2850)

* Migration guide for action/observation changes.

*   remove  whitespace

docs/Migrating.md

 ## Migrating from ML-Agents toolkit v0.11.0
 
 ### Important Changes
+Text actions and observations, and custom action and observation protos have been removed.
 
 ### 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.