cluster: clarify construct Handle

Improve the readability for costruct SharedHandle and
RoundRobinHandle

PR-URL: https://github.com/nodejs/node/pull/37385


Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>