You don't use a hook, you essentially add help, over-riding the core help, in the same way you are adding the NickServ register command that will be used instead of the cores.
something like this:
c = createCommand("REGISTER", do_register, NULL, -1, -1, -1, -1, -1);
moduleAddCommand(NICKSERV, c, MOD_HEAD);
moduleAddHelp(c, ns_help_register);
so, moduleAddHelp is what you're after, you'd then need to write the ns_help_register function, hope that puts you on right path