Add warning about synchronized service with private services

Author: adrienbrault

cookbook/service_container/scopes.rst

@@ -209,6 +209,11 @@ your code. This should also be taken into account when declaring your service:
                 ->setSynthetic(true)
                 ->setSynchronized(true);
 
+.. caution::
+   
+   The service using the synchronized service will need to be public in order have its
+   setter called when the scope changes.
+
 .. _changing-service-scope:
 
 Changing the Scope of your Service