Undocumented predicates

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

 substitute(Arg1, Arg2, Arg3)

 substitute_value(Arg1, Arg2, Arg3, Arg4)

 substitute_value(Arg1, Arg2, Arg3, Arg4, Arg5)

 substitute_values(Arg1, Arg2, Arg3)

 is_subterm(Arg1, Arg2)