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
> after startup or during runtime

I'm super wary of even indirectly hinting that people should use cloudinit beyond boot time (your Chef/Puppet disclaimer later on notwithstanding - in fact maybe you could move that up here).

Also agree with Alex that "by default" is misleading, it'll always happen during built unless they roll their own OEM