explain that we won't run jobs on the login node in this lesson

Closes carpentries-incubator#250.

_episodes/12-cluster.md

@@ -136,8 +136,13 @@ Individual computers that compose a cluster are typically called *nodes* (althou
 hear people call them *servers*, *computers* and *machines*). On a cluster, there are different
 types of nodes for different types of tasks. The node where you are right now is called the *head
 node*, *login node*, *landing pad*, or *submit node*. A login node serves as an access point to the
-cluster. As a gateway, it is well suited for uploading and downloading files, setting up software,
-and running quick tests. It should never be used for doing actual work.
+cluster.
+
+As a gateway, it is well suited for uploading and downloading files, setting up software, and
+running quick tests. Generally speaking, the login node should not be used for time-consuming or
+resource-intensive tasks. You should be alert to this, and check with your site's operators or
+documentation for details of what is and isn't allowed. In these lessons, we will avoid running jobs
+on the head node.
 
 > ## Dedicated Transfer Nodes
 >