Upgrade doc could be more precise about upgrade script arguments

[thomasdanan]

Component:

'doc'

Why this is needed:

When following Upgrade section in doc: https://metal-k8s.readthedocs.io/en/latest/operation/upgrade.html

in Upgrade Pre-requisites section we explain how to launch upgrade in dryrun mode with the following command:

/srv/scality/metalk8s-X.X.X/upgrade.sh --destination-version <destination_version> --dry-run --verbose

It is only in a later chapter that we explain that metalk8s-X.X.X should be the new versions of MetalK8s to which we want to upgrade.
Also, we don't explain clearly how to set destination-version: the administrator may think it is metalK8s-2.4.2 but it is 2.4.2 only.

What should be done:

• Move the Important note (The version prefix metalk8s-X.X.X as used below during a MetalK8s upgrade must be the new MetalK8s version you would like to upgrade to.) before explaining how to launch upgrade in dryrun mode
• Give an example with real MetalK8s version

For any command we document, we should give real example

Implementation proposal (strongly recommended):

Test plan: