I don't think there is much changes as to how string tables work. They still function the same to me. When designing your workflow, you reference the fields in the environment library. When you deploy the workflow, the fields in the environment library copies over to your string table, so any process instances running will use the value from the string table. That's how it is back in 4.6.x, and it should still be the same in K2 Five.
In the old workspace in 4.x, you use to have a seperate string table for each workflow. Now in 5.x it seems you just have the one string table that is shared by all workflows, which I guess is the string table that corresponds to whatever the default environment is? The documentation is not real clear on this.
I agree the whitepaper should be updated to reflect K2 5 UI and architecture.