Regularize declaration markup.
Regularize declaration markup.

This no longer special cases the markup for declarations
that have no docstring. Before it would be:

    div (div.spec div)   # With docstring
    div.spec             # Without docstring

With this patch this becomes:

   div (div.spec div)    # With docstring
   div (div.spec)        # Without docstring

The regularity simplifies stylesheets development.

ocaml-doc/odoc,ocaml-doc/odoc