doc: document that addMembership must be called once in a cluster

Fixes: https://github.com/nodejs/node/issues/12572
Refs: https://github.com/nodejs/node/pull/16240

PR-URL: https://github.com/nodejs/node/pull/23746


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>