fix: NetworkSceneManager add do not load during callback note in xml documentation [MTT-3495]

[NoelStephensUnity]

This pertains to #1942 and was determined that the API documentation needs to be updated to let users know not to start new scene events within a scene event callback. This PR adds the additional XML documentation.

MTT-3495

Testing and Documentation

• Includes additions to existing public API documentation.

[jeffreyrainy]

Should we have (or is there) any mechanism to detect and prevent such scene events ?

[NoelStephensUnity]

Why yes there is!
Both NetworkSceneManager.LoadScene and NetworkSceneManager.UnloadScene return a SceneEventProgressStatus.
If you invoke that in the middle of a scene event NetworkSceneManager will return SceneEventProgressStatus.SceneEventInProgress which tells/reminds the user "You are in the middle of a scene event, let it finish completely before trying to start a new one".
The same thing would happen if you tried to load (n) scenes all at the same time and called NetworkSceneManager.LoadScene within some method. The first call (iteration 0) it would return SceneEventProgressStatus.Started and then for every other call (up to n) it would return SceneEventProgressStatus.SceneEventInProgress.

So, this is PR is just to remind users when they are looking through the API that you can't start a new scene event until any current scene events are still in progress.