Add constraint to SWA API backends noting network isolation not supported

Author: thomasgauvin

articles/static-web-apps/apis-overview.md

@@ -45,6 +45,7 @@ The following constraints apply to all API backends:
 - Route rules for APIs only support [redirects](configuration.md#defining-routes) and [securing routes with roles](configuration.md#securing-routes-with-roles).
 - Only HTTP requests are supported for APIs. WebSocket, for example, is not supported.
 - The maximum duration of each API request 45 seconds.
+- Backends that are network isolated are not supported.
 
 ## Next steps