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.
CoreOS ships a socket unit for fleet (`fleet.socket`) which binds to a Unix domain socket, `/var/run/fleet.socket`. To serve the fleet API over a network address, extend or replace this socket file. For example, writing the following drop-in to `...` would enable fleet to be reached over the local port `49153`: