Show argument names in :doc

Now, :doc will show the names of any documented arguments, even if they
are not used in the body.

Fixes #1000.
Stop showing __pi_arg and MNs in docs
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
Merge pull request #1001 from david-christiansen/doc-arg-names

Show argument names in :doc, and don't show undocumented args