package/python3: change BR2_PACKAGE_PYTHON_PYEXPAT description

This patch changes BR2_PACKAGE_PYTHON3_PYEXPAT description and
help text to underline that all the xml libraries will be
included in python.
It also reorders alphabetically the affected option.

Signed-off-by: Angelo Compagnucci <angelo.compagnucci@gmail.com>
Reviewed-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>