spot::contains(a, b) should test a⊇b.  It was testing a⊆b instead.

* NEWS: Mention the bug.
* spot/twaalgos/contains.cc, spot/twaalgos/contains.hh: Fix the
code and documentation.
* tests/python/contains.ipynb: Adjust description and expected
results.
* python/spot/__init__.py: Also swap the argument of
language_containment_checker.contains()
* bin/autfilt.cc: Adjust usage.