Update the migration/upgrade notes with correct instructions on how to upgrade between MRTK versions.

#5770

The existing instructions weren't quite complete, because when you approached step #6 (re-adding MRTK objects to the scene) there is a strong likelihood that the profiles (and other assets outside of the MRTK) haven't been refreshed in the asset database. As a result, doing things like adding the MRTK to the scene and picking a profile wouldn't pick up existing profiles (because the asset database wouldn't have the correct information.)

This requires forcing Unity to regenerate its asset database - the easiest way to do this is just to close it and delete the Library/ folder (which causes an asset database refresh)

See #5770 (comment) for alternatives considered

Author: wiwei

Documentation/ReleaseNotes.md

@@ -50,7 +50,9 @@ For the smoothest upgrade path, please use the following steps.
     > [!Note]
     > If additional extensions had been installed, they may need to be re-imported.
     - (Optional) Examples
-1. For each scene in the project
+1. Close Unity and Delete the **Library** folder. This step is necessary to force Unity to refresh its
+   asset database and reconcile existing custom profiles.
+1. Launch Unity, and for each scene in the project
     - Delete **MixedRealityToolkit** and **MixedRealityPlayspace**, if present, from the hierarchy
     - Select **MixedRealityToolkit -> Add to Scene and Configure**