Create a doc for COS day-2 charm revisions updated to latest in track

[MichaelThamm]

Given that this feature PR is merged for the product modules:

• feat: juju_charm datasource and a upgrades.tf file #278

we want to update documentation for COS so that our users know:

Previously, if you deploy COS on day 1 and then on day 2, you would always get the same revisions. This is no longer the case and charm revisions might change per apply because they are updated to "latest in track". To pin these revisions, use the revision input per COS component.