Add ref docs for junit_family option

doc/en/reference.rst

@@ -1015,6 +1015,20 @@ passed multiple times. The expected format is ``name=value``. For example::
    This tells pytest to ignore deprecation warnings and turn all other warnings
    into errors. For more information please refer to :ref:`warnings`.
 
+.. confval:: junit_family
+
+    .. versionadded:: 4.2
+
+    Configures the format of the generated JUnit XML file. The possible options are:
+
+    * ``xunit1`` (or ``legacy``): produces old style output, compatible with the xunit 1.0 format. **This is the default**.
+    * ``xunit2``: produces `xunit 2.0 style output <https://github.com/jenkinsci/xunit-plugin/blob/xunit-2.3.2/src/main/resources/org/jenkinsci/plugins/xunit/types/model/xsd/junit-10.xsd>`__,
+        which should be more compatible with latest Jenkins versions.
+
+    .. code-block:: ini
+
+        [pytest]
+        junit_family = xunit2
 
 .. confval:: junit_suite_name