Coverage: don’t require data docs if all fields are documented

[Profpatsch]

When I have a sum type like:

data ParseError
  = SyntaxError Text
    -- ^ the input string was not a syntactically valid nix expression
  | UnknownParseError
    -- ^ catch-all error
  deriving (Show, Eq)

where it does not really make sense to document the thing itself, the coverage still complains.

I suggest that we don’t warn for a data declaration where all fields are documented but the toplevel is not.