Mention in docs that etcd data-dir and wal-dir should not be changed

[brandond]

I have tried moving back etcd data path back to /var/lib/rancher/rke2/server/db and that seem to be working now as expected. I still have to do umask 0022 before exucuting rke2 server --cluster-reset ..., but when it cluster restore / reset is done I did revert it to umask 0027 and rke2-server was able to start up normally without any issues so no need to modify systemd unit.
Thanks for clarifying with etcd data path, I did not see anything in docs saying that custom etcd data path can not be used, maybe worth adding that.

Originally posted by @riuvshyn in rancher/rke2#6466 (comment)