Clarify Issuer URL and token audiences for App Service auth

Author: cephalin

articles/app-service/configure-authentication-provider-aad.md

@@ -80,6 +80,8 @@ To use an existing registration, select either:
 
     You can also configure the application to [use an identity instead of a client secret][fic-config]. Support for using an identity is currently in preview.
   - **Issuer URL**. This URL takes the form `<authentication-endpoint>/<tenant-id>/v2.0`. Replace `<authentication-endpoint>` with the authentication endpoint [value that's specific to the cloud environment](/entra/identity-platform/authentication-national-cloud#azure-ad-authentication-endpoints). For example, a workforce tenant in global Azure would use `https://login.microsoftonline.com` as its authentication endpoint.
+
+      You can find this value in the Microsoft Entra admin center. Go to **App registrations**, select your app, and then select **Endpoints**. Copy the **OpenID Connect metadata document** endpoint for your tenant, and then remove `/.well-known/openid-configuration` from the end of the URL. For example, if the metadata endpoint is `https://login.microsoftonline.com/<tenant-id>/v2.0/.well-known/openid-configuration`, use `https://login.microsoftonline.com/<tenant-id>/v2.0` as the issuer URL.
 
     > [!NOTE]
     > If you created your identity provider using the express setup (Option 1), the issuer URL is automatically set to use the legacy `https://sts.windows.net` endpoint. To align with current Microsoft Entra ID best practices, edit your identity provider and update the issuer URL to use `https://login.microsoftonline.com/<tenant-id>/v2.0` instead.
@@ -169,7 +171,7 @@ To use an existing registration, select **Provide the details of an existing app
 
 - **Application (client) ID**
 - **Client secret**
-- **Issuer URL**
+- **Issuer URL**. In the Microsoft Entra admin center, go to **App registrations**, select your app, and then select **Endpoints**. Copy the **OpenID Connect metadata document** endpoint for your tenant, and then remove `/.well-known/openid-configuration` from the end of the URL. For example, if the metadata endpoint is `https://login.microsoftonline.com/<tenant-id>/v2.0/.well-known/openid-configuration`, use `https://login.microsoftonline.com/<tenant-id>/v2.0` as the issuer URL.
 
 If you need to manually create an app registration in an external tenant, see [Register an app in your external tenant](/entra/external-id/customers/how-to-register-ciam-app?tabs=webapp#register-your-web-app).
 
@@ -219,6 +221,8 @@ For **Tenant requirement**, choose whether to:
 - Allow requests from specific tenants.
 - Use default restrictions based on the app registration's tenant.
 
+For **Allowed token audiences**, add any audience values that your app should accept in the `aud` claim of incoming access tokens. You commonly need this setting when clients request tokens by using the app registration's **Application ID URI**, such as `api://<application-client-id>` or a custom URI like `https://contoso.com/api`. The app registration's client ID is already accepted by default, so you typically add values here only if your app accepts another audience format.
+
 Your app might still need to make other authorization decisions in code. For more information, see [Use a built-in authorization policy](#use-a-built-in-authorization-policy) later in this article.
 
 ## Configure authentication settings