📖 Document how to upgrade a multi-tenant cluster

[fabriziopandini]

What this PR does / why we need it:
This PR improves the clusterctl upgrade docs by documenting how to upgrade a multi-tenant cluster where each provider instance was installed with different env variables

Which issue(s) this PR fixes:
Fixes #2997

/cc @MaxRink

[k8s-ci-robot]

@fabriziopandini: GitHub didn't allow me to request PR reviews from the following users: MaxRink.

Note that only kubernetes-sigs members and repo collaborators can review this PR, and authors cannot review their own PRs.

In response to this:

What this PR does / why we need it:
This PR improves the clusterctl upgrade docs by documenting how to upgrade a multi-tenant cluster where each provider instance was installed with different env variables

Which issue(s) this PR fixes:
Fixes #2997

/cc @MaxRink

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[wfernandes]

Just a suggestion: Could the steps be numbered to show the ordered operations?

[wfernandes]

/lgtm

[vincepri]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: vincepri

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [vincepri]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment