Mention client level user configs

website/source/docs/job-specification/task.html.md

@@ -65,7 +65,9 @@ job "docs" {
 - `user` `(string: <varies>)` - Specifies the user that will run the task.
   Defaults to `nobody` for the [`exec`][exec] and [`java`][java] drivers.
   [Docker][] and [rkt][] images specify their own default users.  This can only
-  be set on Linux platforms.
+  be set on Linux platforms, and clients can restrict
+  [which drivers][user_drivers] are allowed to run tasks as
+  [certain users][user_blacklist].
 
 - `vault` <code>([Vault][]: nil)</code> - Specifies the set of Vault policies
   required by the task. This overrides any `vault` block set at the `group` or
@@ -165,3 +167,5 @@ task "server" {
 [java]: /docs/drivers/java.html "Nomad Java Driver"
 [Docker]: /docs/drivers/docker.html "Nomad Docker Driver"
 [rkt]: /docs/drivers/rkt.html "Nomad rkt Driver"
+[user_drivers]: /docs/agent/configuration/client.html#_quot_user_checked_drivers_quot_
+[user_blacklist]: /docs/agent/configuration/client.html#_quot_user_blacklist_quot_