Improve documentation around custom error page on App Service

I recognize that [Custom Error Pages](https://github.com/Azure/app-service-linux-docs/blob/master/HowTo/configure_custom_error_page.md) is a preview feature. More documentation around this feature would be incredibly useful. The IP Restriction example is extremely helpful, but not much other information exists. A couple of questions:

- In what scenarios will the custom error page return?
- Will the custom error page intercept the status code and instead return the custom error page?
- At what layer is this custom error page set up (IIS?)? Why don't I see the custom error page when my application is in a `stopped` state?

Thank you in advance!