Introduce `signature_element` and use it to simplify `signature`. `signature_nodoc` disappears.
Introduce `signature_element` and use it to simplify `signature`.
`signature_nodoc` disappears.

gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml