Document new min_depth configuration option

Author: james-johnston-thumbtack

reference/configuration/debug.rst

@@ -19,6 +19,7 @@ Configuration
 -------------
 
 * `max_items`_
+* `min_depth`_
 * `max_string_length`_
 * `dump_destination`_
 
@@ -30,6 +31,19 @@ max_items
 This is the maximum number of items to dump. Setting this option to ``-1``
 disables the limit.
 
+min_depth
+~~~~~~~~~
+
+**type**: ``integer`` **default**: ``1``
+
+Configures the minimum tree depth where we are guaranteed to clone
+all the items. After this depth is reached, only ``max_items``
+items will be cloned. The default value is ``1``, which is consistent
+with older Symfony versions.
+
+.. versionadded:: 3.4
+    The ``min_depth`` option was introduced in Symfony 3.4.
+
 max_string_length
 ~~~~~~~~~~~~~~~~~