Introduce missing remark as commented in original PR

https://github.com/pytest-dev/pytest/pull/6870/files#r390667966

CHANGELOG.rst

@@ -26,7 +26,7 @@ Features
 
 - `#6870 <https://github.com/pytest-dev/pytest/issues/6870>`_: New ``Config.invocation_args`` attribute containing the unchanged arguments passed to ``pytest.main()``.
 
-
+  Remark: while this is technically a new feature and according to our `policy <https://docs.pytest.org/en/latest/py27-py34-deprecation.html#what-goes-into-4-6-x-releases>`_ it should not have been backported, we have opened an exception in this particular case because it fixes a serious interaction with ``pytest-xdist``, so it can also be considered a bugfix.
 
 Trivial/Internal Changes
 ------------------------