Added content to port redirect doc

docs/sources/use/port_redirection.rst

@@ -6,18 +6,20 @@
 Port redirection
 ================
 
+Docker can redirect public tcp ports to your container, so it can be reached over the network.
 Port redirection is done on ``docker run`` using the -p flag.
 
-Here are the 3 ways to redirect a port:
+A port redirect is specified as PUBLIC:PRIVATE, where tcp port PUBLIC will be redirected to
+tcp port PRIVATE. As a special case, the public port can be omitted, in which case a random
+public port will be allocated.
 
 .. code-block:: bash
 
-    # the port 80 in the container is mapped to a random port of the host
+    # A random PUBLIC port is redirected to PRIVATE port 80 on the container
     docker run -p 80 <image> <cmd>
 
-    # the port 80 in the container is mapped to the port 80 of the host
-    docker run -p :80 <image> <cmd>
+    # PUBLIC port 80 is redirected to PRIVATE port 80
+    docker run -p 80:80 <image <cmd>
 
-    # the port 80 in the container is mapped to the port 5555 of the host
-    docker run -p 5555:80 <image <cmd>
 
+Default port redirects can be built into a container with the EXPOSE build command.