Undocumented predicates

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

 assert_arity(Arg1, Arg2)

 bad_arity(Arg1, Arg2)

 ensure_arity(Arg1, Arg2)

 functor_check_univ(Arg1, Arg2, Arg3)

 get_arity(Arg1, Arg2, Arg3)

 mpred_prop_file_begin

 xlisting(Arg1)