Updated environment description

.. to make it even more clear that this env variables are there because of the --link option.

Docker-DCO-1.1-Signed-off-by: Matthias Kühnle <git.nivoc@neverbox.com> (github: float64)

docs/sources/examples/running_redis_service.rst

@@ -67,7 +67,8 @@ Once inside our freshly created container we need to install Redis to get the
     apt-get -y install redis-server
     service redis-server stop
 
-Docker sets some environment variables in our web application container. 
+Since we used the ``--link redis:db`` option before. Docker has created some 
+environment variables in our web application container. 
 Let's inspect them with the ``env`` command.
 
 .. code-block:: bash