upstream?

Have not done the paperwork for talking to GNU upstream.

Doesn't this already follow from the use of singular form in

   Directive: %parse-param {ARGUMENT-DECLARATION} ...

and the example that uses two arguments with separate braces:

   %parse-param {int *nastiness} {int *randomness}

?  I guess being more explicit doesn't hurt, though.

It's kind of 'if you know how it is you can see it there' thing.

As in if you have no idea how it's implemented internally it's not clear at all.

The other option would be bison refusing multiple arguments in one %parse-param but it does not. Documenting the deficiency sounds easier than fixing it.