[Contracts][DependencyInjection] Mention that locators are traversable

Author: alexandre-daubois

service_container/service_subscribers_locators.rst

@@ -110,18 +110,30 @@ in the service subscriber::
     that you have :ref:`autoconfigure <services-autoconfigure>` enabled. You
     can also manually add the ``container.service_subscriber`` tag.
 
-The injected service is an instance of :class:`Symfony\\Component\\DependencyInjection\\ServiceLocator`
-which implements both the PSR-11 ``ContainerInterface`` and :class:`Symfony\\Contracts\\Service\\ServiceProviderInterface`.
-It is also a callable and a countable::
+The injected service is an instance of
+:class:`Symfony\\Component\\DependencyInjection\\ServiceLocator`. A service
+locator is a container that contains a set of services, but only instantiates
+them when they are actually used. The service locator is countable and
+traversable::
 
     // ...
     $numberOfHandlers = count($this->locator);
     $nameOfHandlers = array_keys($this->locator->getProvidedServices());
+
+    // you can iterate through all services of the locator
+    foreach ($this->locator as $serviceId => $service) {
+        // do something with the service, the service id or both
+    }
+
     // ...
-    $handler = ($this->locator)($commandClass);
+    $handler = $this->locator->get($commandClass);
 
     return $handler->handle($command);
 
+.. versionadded:: 7.1
+
+    Service locators are traversable since Symfony 7.1.
+
 Including Services
 ------------------