Undocumented predicates

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

 ::(Arg1, Arg2)

 le(Arg1, Arg2)

 lt(Arg1, Arg2)

 plus(Arg1, Arg2, Arg3)

 ne(Arg1, Arg2)

 ge(Arg1, Arg2)

 gt(Arg1, Arg2)

 alldistinct(Arg1)