doxygen: improve formula documentation
* doc/Doxyfile.in: Adjust to hide SPOT_API.
* doc/mainpage.dox: Adjust like to parse_infix_psl().
* src/tl/formula.hh: Document most methods of formula,
and fix the definition of the comparisons operator.
* src/ltlparse/public.hh, src/tl/apcollect.hh, src/tl/declenv.hh,
src/tl/defaultenv.hh, src/tl/dot.hh, src/tl/environment.hh,
src/tl/length.hh, src/tl/mark.hh, src/tl/nenoform.hh, src/tl/print.hh,
src/tl/randomltl.hh, src/tl/relabel.hh, src/tl/simpfg.hh,
src/tl/simplify.hh, src/tl/unabbrev.hh: Adjust doxygen group.
dcb9d7e8a8