Haddockumentation of `Distribution.FieldGrammar.Class` out of sync with code

The doc mentions parameters `s` and `a`, but the new parameters are `c` and `g` (and likely have other roles).
https://github.com/haskell/cabal/blob/9a104a99c50e451731d9650e6c0f50721d7d1ce1/Cabal-syntax/src/Distribution/FieldGrammar/Class.hs#L30-L42

Broken by 920d6a98ebae2a12e9af1733e27c69e0becb4547.
@phadej, what would the updated doc be?

(Context: migrating `hackage-server` to Cabal 3.4.)

Open Cabal-syntax/src/Distribution/FieldGrammar/Class.hs around lines 30–42 and compare the Haddock parameter documentation with the current declarations. Update the documentation to describe c and g and their roles accurately; done means the parameter names and descriptions match the code.

Written by the indexing model from the issue text.