docs: NetworkSceneManager, SceneEventType, SceneEvent, SceneEventProgressStatus

[NoelStephensUnity]

This is an update to the xml documentation in order to get better formatting with a few minor changes in the text copy.

PR Checklist

• Have you added a backport label (if needed)? For example, the type:backport-release-* label. After you backport the PR, the label changes to stat:backported-release-*.
• Have you updated the changelog? Each package has a CHANGELOG.md file.
• Have you updated or added the documentation for your PR? When you add a new feature, change a property name, or change the behavior of a feature, it's best practice to include related documentation changes in the same PR or a link to the documenation repo PR if this is a manual update.

Changelog

com.unity.netcode.gameobjects

• Added: The package whose Changelog should be added to should be in the header. Delete the changelog section entirely if it's not needed.

Testing and Documentation

• No tests have been added.
• Includes edits to existing public API documentation.