kubeadm-reset: add notes about more manual cleanup steps

[neolit123]

currently the kubeadm reset command when finished prints the following:

The reset process does not clean CNI configuration. To do so, you must remove /etc/cni/net.d

The reset process does not reset or clean up iptables rules or IPVS tables.
If you wish to reset iptables, you must do so manually by using the "iptables" command.

If your cluster was setup to utilize IPVS, run ipvsadm --clear (or similar)
to reset your system's IPVS tables.

The reset process does not clean your kubeconfig files and you must remove them manually.
Please, check the contents of the $HOME/.kube/config file.

instead of printing that information from the CLI we should keep it document at the website and link to the reset page from the output of the command.

xref

• improve "kubeadm reset" kube-proxy cleanup kubeadm#3133
• Document kube-proxy --cleanup #49445
• kube-proxy --cleanup issues kubernetes#129639

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[neolit123]

/cc @SataQiu @pacoxu @carlory
/cc @danwinship (for the kubeproxy cleanup part)

[neolit123]

changing 'traffic assets' to 'traffic rules', despite IPVS using 'tables', afaik.

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