Sorted: mention stability

[wstomv]

Mention that sorting is stable:

• https://python-reference.readthedocs.io/en/latest/docs/functions/sorted.html
• https://python-reference.readthedocs.io/en/latest/docs/list/sort.html

This is an important property to know when using these sorting functions.

Also see: https://docs.python.org/3/howto/sorting.html