fix unclear documentation for docker container

the docker container module's `exposed_ports` was slightly ambigous.

Use the official Docker documentation to define what an `exposed port`
is.

Resolves: ansible/ansible-modules-core#5303
Signed-off-by: Daniel Andrei Minca <mandrei17@gmail.com>

cloud/docker/docker_container.py

@@ -124,7 +124,8 @@ options:
     required: false
   exposed_ports:
     description:
-      - List of additional container ports to expose for port mappings or links.
+      - List of additional container ports which informs Docker that the container
+        listens on the specified network ports at runtime.
         If the port is already exposed using EXPOSE in a Dockerfile, it does not
         need to be exposed again.
     default: null