tweak docs
Changed `define-union-language' to merge nonterminals and their
right-hand-sides instead of causing an error when more than one language
in the union defines the same nonterminal.
follow up to William J. Bowman's commit

- put the tests together with the other define-union-language tests
- fix the docs
- add a release note
- construct the merged language a bit more directly
- properly deal with this kind of thing:
  (define-language L1 (e f ::= 1 2 3))
  (define-language L2 (e g ::= 4 5 6))
  (define-union-language L L1 L2)
used the wrong dir when checking the info.rkt santity

thanks to Jos Koot for pointing out this bug