This follows up on a mail from Sonali Dutta.

* src/tgba/bdddict.hh (assert_emptiness, ~bdd_dict): Better
documentation.
* src/tgba/formula2bdd.hh (formula_to_bdd): Mention
unregister_all_my_variables().
(bdd_to_formula): Complete the documentation.
* THANKS: Add Sonali Dutta.