Document that ENV vars are not automatically updated

Unlike the entries in `/etc/hosts`, environment-variables for linked
containers are not automatically updated if the linked container is
restarted.

This adds a note to the documentation in;
https://docs.docker.com/userguide/dockerlinks/#environment-variables
and
https://docs.docker.com/reference/run/#env-environment-variables

To make users aware that this is the case and recommends them to use
the `/etc/hosts` entries in stead.

I added this change because users were expecting environment variables
to be updated automatically as well (https://github.com/docker/docker/issues/10164).

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

docs/sources/reference/run.md

@@ -647,6 +647,12 @@ mechanism to communicate with a linked container by its alias:
 If you restart the source container (`servicename` in this case), the recipient
 container's `/etc/hosts` entry will be automatically updated.
 
+> **Note**:
+> Unlike host entries in the `/ets/hosts` file, IP addresses stored in the
+> environment variables are not automatically updated if the source container is
+> restarted. We recommend using the host entries in `/etc/hosts` to resolve the
+> IP address of linked containers.
+
 ## VOLUME (shared filesystems)
 
     -v=[]: Create a bind mount with: [host-dir]:[container-dir]:[rw|ro].