remove the -D proxy option from the docs

It doesn't do anything useful since the weave script needs to be able
to talk to docker on the standard unix socket.

Also remove dangling reference to 'localhost', and move note about
firwalls to where it is most pertinent.

Part fix to weaveworks#972.

site/proxy.md

@@ -29,17 +29,14 @@ To start the proxy, run
 
     host1$ weave launch-proxy
 
-By default, the proxy will connect to docker at
-`unix:///var/run/docker.sock`. This can be adjusted with the `-D`
-argument, e.g.
-
-    host1$ weave launch-proxy -D tcp://127.0.0.1:4243
-
 By default, the proxy listens on port 12375, on all network
 interfaces. This can be adjusted with the `-H` argument, e.g.
 
     host1$ weave launch-proxy -H tcp://127.0.0.1:9999
 
+If you are working with a remote docker daemon, then any firewalls
+inbetween need to be configured to permit access to the proxy port.
+
 All docker commands can be run via the proxy, so it is safe to adjust
 your `DOCKER_HOST` to point at the proxy. Weave provides a convenient
 command for this:
@@ -52,11 +49,6 @@ Alternatively, the proxy host can be set on a per-command basis with
 
     host1$ docker $(weave proxy-config) ps
 
-If you are working with a remote docker daemon, then `localhost` in
-the above needs to be replaced with the docker daemon host, and any
-firewalls inbetween need to be configured to permit access to
-the proxy port.
-
 The proxy can be stopped with
 
     host1$ weave stop-proxy