Update serviceprincipal.md - Add beta

Author: FaithOmbongi

api-reference/beta/resources/serviceprincipal.md

@@ -104,7 +104,7 @@ This resource supports using [delta query](/graph/delta-query-overview) to track
 |notificationEmailAddresses|String collection|Specifies the list of email addresses where Microsoft Entra ID sends a notification when the active certificate is near the expiration date. This is only for the certificates used to sign the SAML token issued for Microsoft Entra Gallery applications.|
 |passwordCredentials|[passwordCredential](passwordcredential.md) collection|The collection of password credentials associated with the service principal. Not nullable. |
 |passwordSingleSignOnSettings|[passwordSingleSignOnSettings](passwordsinglesignonsettings.md)|The collection for settings related to password single sign-on. Use `$select=passwordSingleSignOnSettings` to read the property. Read-only for [applicationTemplates](applicationTemplate.md) except for custom applicationTemplates. |
-|preferredSingleSignOnMode|string|Specifies the single sign-on mode configured for this application. Microsoft Entra ID uses the preferred single sign-on mode to launch the application from Microsoft 365 or the Microsoft Entra My Apps. The supported values are `password`, `saml`, `notSupported`, and `oidc`.|
+|preferredSingleSignOnMode|string|Specifies the single sign-on mode configured for this application. Microsoft Entra ID uses the preferred single sign-on mode to launch the application from Microsoft 365 or the Microsoft Entra My Apps. The supported values are `password`, `saml`, `notSupported`, and `oidc`. **Note:** This field might be `null` for older SAML apps and for OIDC applications where it isn't set automatically.|
 |permissionGrantPreApprovalPolicies|[permissionGrantPreApprovalPolicy](../resources/permissiongrantpreapprovalpolicy.md) collection|The list of preapproval policies that has been assigned to the service principal.|
 |preferredTokenSigningKeyEndDateTime|DateTimeOffset|Specifies the expiration date of the keyCredential used for token signing, marked by **preferredTokenSigningKeyThumbprint**. Updating this attribute isn't currently supported. For details, see [ServicePrincipal property differences](/graph/migrate-azure-ad-graph-property-differences#serviceprincipal-property-differences).|
 |preferredTokenSigningKeyThumbprint|String|This property can be used on SAML applications (apps that have **preferredSingleSignOnMode** set to `saml`) to control which certificate is used to sign the SAML responses. For applications that aren't SAML, don't write or otherwise rely on this property. |