Add support for multi-arity links

[wouterbeek]

Predicates that only differ in their arity are often documented together, e.g.:

%! rdf(?S, ?P, ?O)
%! rdf(?S, ?P, ?O, ?G)

It would be nice if multi-arity predicate letter notation, e.g., rdf/[3,4], would introduce a link to the documentation of rdf/3.