DOC-4411 RS: Add step to avoid removing a primary node last during rolling upgrade (#980)

Author: rrelledge

content/operate/rs/installing-upgrading/upgrading/upgrade-os.md

@@ -80,7 +80,7 @@ To upgrade the cluster's operating system, use one of the following rolling upgr
 
 1. [Remove one node]({{< relref "/operate/rs/clusters/remove-node#remove-a-node" >}}) running the earlier OS version from the cluster.
 
-1. Repeat the previous steps until all nodes with the earlier OS version are removed.
+1. Repeat the previous steps until all nodes with the earlier OS version are removed. If the final node to remove from the cluster is the primary node, [demote it]({{<relref "/operate/rs/clusters/change-node-role#demote-primary-node">}}) to a secondary node before you remove it.
 
 ### Replace node upgrade method {#replace-node-upgrade}
 
@@ -144,4 +144,4 @@ To upgrade the cluster's operating system, use one of the following rolling upgr
         ...
         ```
 
-1. Repeat the previous steps until all nodes with the earlier OS version are replaced.
+1. Repeat the previous steps until all nodes with the earlier OS version are replaced. If the final node to remove from the cluster is the primary node, [demote it]({{<relref "/operate/rs/clusters/change-node-role#demote-primary-node">}}) to a secondary node before you remove it.