Only show documented arguments in :doc

This reduces redundancy, avoids showing internal names like __pi_arg,
and takes up less space. New output:

    Idris> :doc foldl
    foldl : Foldable t => (acc -> elt -> acc) -> acc -> (t elt) -> acc

    Idris> :doc id
    id : a -> a
        Identity function.

    Idris> :doc the
    the : (a : Type) -> (x : a) -> a
        Manually assign a type to an expression.
        Arguments:
            a : Type  -- the type to assign

            x : a  -- the element to get the type