update changelog and link to migration (#3893)

com.unity.ml-agents/CHANGELOG.md

 - The SideChannel API has changed (#3833, #3660) :
   - Introduced the `SideChannelManager` to register, unregister and access side
   channels.
-  - `EnvironmentParameters` replaces the default `FloatProperties`.
-  You can access the `EnvironmentParameters` with
-  `Academy.Instance.EnvironmentParameters` on C# and create an
-  `EnvironmentParametersChannel` on Python
+  - `Academy.FloatProperties` was replaced by `Academy.EnvironmentParameters`.
+  See the [Migration Guide](../docs/Migrating.md) for more details on upgrading.
   - `SideChannel.OnMessageReceived` is now a protected method (was public)
   - SideChannel IncomingMessages methods now take an optional default argument,
   which is used when trying to read more data than the message contains.
-- CameraSensorComponent.m_Grayscale and RenderTextureSensorComponent.m_Grayscale
+- `CameraSensorComponent.m_Grayscale` and `RenderTextureSensorComponent.m_Grayscale`
   were changed from `public` to `private`. These can still be accessed via their
   corresponding properties. (#3808)
 - Public fields and properties on several classes were renamed to follow Unity's

docs/Migrating.md

   channels.
   - `EnvironmentParameters` replaces the default `FloatProperties`.
   You can access the `EnvironmentParameters` with
-  `Academy.Instance.EnvironmentParameters` on C# and create an
-  `EnvironmentParametersChannel` on Python
+  `Academy.Instance.EnvironmentParameters` on C#. If you were previously creating
+   a `UnityEnvironment` in python and passing it a `FloatPropertiesChannel`,
+   create an `EnvironmentParametersChannel` instead.
   - `SideChannel.OnMessageReceived` is now a protected method (was public)
   - SideChannel IncomingMessages methods now take an optional default argument,
   which is used when trying to read more data than the message contains.