Update upgrades.md with serial=1 for rolling updates

[titansmc]

modify env serial to have real rolling upgrades

What type of PR is this?

/kind documentation

What this PR does / why we need it:
Clear explanation of rolling upgrade

Update upgrades.md with serial=1 for rolling updates

[k8s-ci-robot]

Hi @titansmc. Thanks for your PR.

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[yankay]

/ok-to-test

[VannTen]

Uh, I though this was documented...
It does not fit in that section (Node-based upgrade) though, it should either be in "Graceful upgrade" or in docs/vars.md.
More importantly, "one node at a time" is just a just a special case.
I think we should document that this is just the Ansible "serial" applied to some of our plays (and link to that in Ansible docs) or explicit point out what it does.

[floryut]

@titansmc Thank you, approving the idea of the PR, waiting on lgtm when final version is ready 😉

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: floryut, titansmc

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[yankay]

Thanks a lot @titansmc
/lgtm