doc: move fleet API config info

Now that we ship an actual socket unit in CoreOS, it's not necessary to tell people how to enable enable the API manually. It is still useful to inform users how to bind on additional IP addresses, though.
I know, but you need to make sure fleet.service is stopped before you start fleet.socket. Once fleet.socket is back up, you want to start fleet.service immediately rather than waiting on it to get socket-activated, since the agent needs to be running. We could play with the Requires/BindsTo params in our units on CoreOS to tie the units together a bit closer than they are today if that would help.