Undocumented predicates

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

 gen_argument_chains(Arg1, Arg2)

 argument_chain(Arg1, Arg2)

 unlinked_arg(Arg1, Arg2, Arg3, Arg4)

 arg_id(Arg1, Arg2, Arg3, Arg4, Arg5, Arg6)

 lead_to_root(Arg1)

 linked_arg(Arg1, Arg2)