Add subsubsection

Author: sebastianbergmann

src/configuration.rst

@@ -619,6 +619,11 @@ A required PHP version can be specified using the ``phpVersion`` and ``phpVersio
 
 In the example above, the tests from the ``tests/unit`` directory are only added to the test suite if the PHP version is at least 8.0.0. The ``phpVersionOperator`` attribute is optional and defaults to ``>=``.
 
+The ``groups`` Attribute
+************************
+
+Possible values: string
+
 The tests that are found using ``<directory>`` and ``<file>`` elements can be added to a comma-separated list of groups:
 
 .. code-block:: xml