gh-140793: Improve documentatation and tests for the ensure_ascii option in the json module (GH-140906)

* Document that ensure_ascii=True forces escaping not only non-ASCII, but also
  non-printable characters (the only affected ASCII character is U+007F).
* Ensure that the help output for the json module does not exceed 80
  columns (except one long line in an example and generated lines).
* Add more tests.