docs: add comment about implementation

Author: lsipii

VirtualFinland.UserAPI/src/VirtualFinland.UsersAPI/Security/Features/SecurityFeature.cs

@@ -131,6 +131,7 @@ public virtual Task ValidateSecurityTokenAudienceByStaticConfiguration(string au
 
     /// <summary>
     /// Validates the token audience by external service
+    /// Implement this in the derived class if using the audience guard service feature
     /// </summary>
     /// <param name="audience"></param>
     /// <exception cref="NotAuthorizedException"></exception>