This addresses a few points from #479.
* doc/org/ltlsynt.tex: New file.
* doc/Makefile.am: Add it.
* doc/org/ltlsynt.org: Show the architecture, and mention more
options.
* bin/spot-x.cc: Document ltlsynt's -x options.
* bin/ltlsynt.cc: Fix default value of --aiger, and typo in its
documentation.
