Fixes #560.

* spot/parseaut/parseaut.yy: Add more comments about handling
of prop_universal in present of multiple initial states.  It took
me time to figure out that it was done correctly.  Also only
reset prop_complete() in the case an initial state is reused.
* tests/core/det.test: Add a test case for the deterministic property.
* tests/python/parsetgba.py: Add tests for complete.
* doc/org/hoa.org: Add more text about the effect of fusing initial
states.
* doc/org/concepts.org (properties): Replace "deterministic" by
"universal".  The former was obsoleted in Spot 2.4.