(AzureCXP) fixes MicrosoftDocs/azure-docs-pr #542950

Added line 37
> If you are hosting custom error pages in Azure Blob Storage, the storage account must be accessible directly from the internet (public access) so that Application Gateway can fetch the pages. Custom error pages cannot be served from a CDN endpoint in front of the storage account — Application Gateway does not currently support fetching error pages from CDN URLs. If you are using a CDN with your storage account, make sure the Application Gateway can access the storage account directly, not only through the CDN.

Author: RNareddy-MSFT

articles/application-gateway/custom-error.md

@@ -34,6 +34,7 @@ The custom error pages are supported for the following response codes:
 | 504 | Gateway timeout |
 
 > [!Note]
+> If you are hosting custom error pages in Azure Blob Storage, the storage account must be accessible directly from the internet (public access) so that Application Gateway can fetch the pages. Custom error pages cannot be served from a CDN endpoint in front of the storage account — Application Gateway does not currently support fetching error pages from CDN URLs. If you are using a CDN with your storage account, make sure the Application Gateway can access the storage account directly, not only through the CDN.
 > - API version 2022-09-01 or later should be used to configure error pages for all available response codes.
 > - Response code 404 (Page not found) is currently unsupported.
 > - The custom error pages are displayed for response codes generated by Application Gateway. If an error originates from the backend server, it is passed along unmodified to the client.