Documentation: Constraints on Parser Monad

I was trying to figure out how to use a monad with a constraint (for MTL or free monad style monads), and came across [this Reddit comment](https://www.reddit.com/r/haskell/comments/709q2t/comment/dn1ugbk/?utm_source=share&utm_medium=web2x&context=3) which mentions that it's supported. I've been unable to find any mention of this in the documentation, is it just outdated in that regard?

Start by locating Happy's existing parser-monad documentation and checking the behavior described in the linked Reddit comment. Determine whether constrained monads are supported, then document the relevant usage and make the support or limitation clear to readers.

Written by the indexing model from the issue text.