Adding anonymous

I'm not sure about `~` being the default - please double-check.

The info is taken from https://symfony.com/blog/new-in-symfony-4-4-lazy-firewalls

Author: ThomasLandauer

reference/configuration/security.rst

@@ -32,6 +32,7 @@ Configuration
 * `erase_credentials`_
 * `hide_user_not_found`_
 * `session_fixation_strategy`_
+* `anonymous`_
 
 **Advanced Options**:
 
@@ -61,6 +62,19 @@ If ``true``, the user is asked to authenticate before each call to the
 ``isGranted()`` method in services and controllers or ``is_granted()`` from
 templates.
 
+anonymous
+~~~~~~~~~
+
+**type**: ``string`` **default**: ``~``
+
+When set to ``lazy``, Symfony loads the user (and starts the session) only if the
+application actually accesses the `User` object (e.g. via a `is_granted()` call in
+a template or `isGranted()` in a controller or service.
+
+.. versionadded:: 4.4
+
+    The option ``lazy`` was introduced in Symfony 4.4.
+
 erase_credentials
 ~~~~~~~~~~~~~~~~~