Improve documentation around custom error page on App Service

[eric-gonzalez-tfs]

I recognize that Custom Error Pages 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!

[eric-gonzalez-tfs]

Any insight would be very much appreciated @jeffwmartinez