[3.8] bpo-34160: explain how to deal with attribute order in ElementTree (GH-14867) (GH-14935)

* Fix the formatting in the documentation of the tostring() functions.

* bpo-34160: Document that the tostring() and tostringlist() functions also preserve the attribute order now.

* bpo-34160: Add an explanation of how users should deal with the attribute order.
(cherry picked from commit a3697db0102b9b6747fe36009e42f9b08f0c1ea8)

Co-authored-by: Stefan Behnel <stefan_ml@behnel.de>
1 file changed