fix(docs): Clarify Desktop URL listening behavior in appsettings.json

Resolves #1875

Author: rnwood

Rnwood.Smtp4dev/appsettings.json

@@ -34,6 +34,7 @@
     // For example, http://localhost:123. 
     // Use `*` in place of hostname to listen for requests on any IP address or hostname using the specified port and protocol (for example, http://*:5000).
     // Separate multiple values with ;
+    // Does not apply to Smtp4Dev Desktop - that always listens on localhost only and the port is chosen automatically.
     "Urls": "http://localhost:5000",
 
     // Specifies the virtual path from web server root where SMTP4DEV web interface will be hosted. e.g. "/" or "/smtp4dev"
@@ -357,4 +358,4 @@
     }
 
 
-  }
+  }