Documentation: Remove reference to PUBLISH Dockerfile directive in networking docs (#4863)

engine/userguide/networking/index.md

@@ -525,16 +525,16 @@ network and user-defined bridge networks.
 
 - You expose ports using the `EXPOSE` keyword in the Dockerfile or the
   `--expose` flag to `docker run`. Exposing ports is a way of documenting which
-  ports are used, but does not actually map or open any ports. Exposing ports
+  ports are used, but **does not actually map or open any ports**. Exposing ports
   is optional.
-- You publish ports using the `PUBLISH` keyword in the Dockerfile or the
+- You publish ports using the `--publish` or `--publish-all` flag to `docker run`. 
-  `--publish` flag to `docker run`. This tells Docker which ports to open on the
+  This tells Docker which ports to open on the container's network interface. 
-  container's network interface. When a port is published, it is mapped to an
+  When a port is published, it is mapped to an
   available high-order port (higher than `30000`) on the host machine, unless
   you specify the port to map to on the host machine at runtime. You cannot
-  specify the port to map to on the host machine in the Dockerfile, because
+  specify the port to map to on the host machine when you build the image (in the
-  there is no way to guarantee that the port will be available on the host
+  Dockerfile), because there is no way to guarantee that the port will be available
-  machine where you run the image.
+  on the host machine where you run the image.
 
   This example publishes port 80 in the container to a random high
   port (in this case, `32768`) on the host machine. The `-d` flag causes the