Fix#198. Reported by Maximilien Colange.
* spot/twaalgos/strength.cc (is_terminal): Test that no accepting
transition lead to a rejecting SCC.
* tests/core/strength.test: Add test case.
* spot/twaalgos/strength.hh, spot/twa/twa.hh, doc/org/concepts.org:
Adjust documentation.
* NEWS: Mention the fix.