Miscellaneous improvements to the behavior and documentation of Visitor, Transformer, Interpreter, and friends (#1543) * tweak docs * add missing docstrings, have decorators error when used incorrectly * add tests for changed behavior * remove implementation of visit_topdown * revert changes to _visitor_args_dec * remove associated tests * remove more bad tests i added also tweak new test for decorator use * factor repeated test * Small phrasing update --------- Co-authored-by: Erez Shinan <erezshin+git@gmail.com>