fix(docs): clarity around boot behavior and unit usage

> I wish this page This page should discuss the difference between using cloud-config on each boot vs using it at first boot to create persistent units; currently it confabulates these two cases

and 
> I wish this page Was clearer about whether cloud-config is executed on only first boot or on every boot
The only reason I was mentioning this is that many people on the mailing list have figured out that you can run it manually after boot. Seems like there is a need for it.