Undocumented predicates

The following predicates are exported, but not or incorrectly documented.

 add_element(Arg1, Arg2, Arg3)

 del_element(Arg1, Arg2, Arg3)

 disjoint(Arg1)

 disjoint(Arg1, Arg2)

 intersect(Arg1, Arg2)

 intersect(Arg1, Arg2, Arg3)

 listtoset(Arg1, Arg2)

 memberchk(Arg1, Arg2)

 pairfrom(Arg1, Arg2, Arg3, Arg4)

 select(Arg1, Arg2, Arg3)

 seteq(Arg1, Arg2)

 subset(Arg1, Arg2)

 subtract(Arg1, Arg2, Arg3)

 symdiff(Arg1, Arg2, Arg3)

 union(Arg1, Arg2, Arg3)