DOC-5159 RS: Added DNS record update step to cluster and OS upgrade instructions (#1571)

Author: rrelledge

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

@@ -114,6 +114,8 @@ To perform a rolling upgrade of the cluster, use one of the following methods:
 
 1. [Add the new node]({{< relref "/operate/rs/clusters/add-node" >}}) to the cluster.
 
+1. If the [cluster uses DNS]({{<relref "/operate/rs/networking/cluster-dns">}}), add the new node’s IP address to the DNS records.
+
 1. [Promote the first new node]({{<relref "/operate/rs/clusters/change-node-role/#promote-secondary-node">}}) to become the primary node.
 
 1. [Remove one node]({{< relref "/operate/rs/clusters/remove-node#remove-a-node" >}}) running the earlier Redis Software version from the cluster.
@@ -142,6 +144,8 @@ To perform a rolling upgrade of the cluster, use one of the following methods:
     rladmin cluster join nodes <cluster_member_ip_address> username <username> password <password> replace_node <node_id>
     ```
 
+1. If the [cluster uses DNS]({{<relref "/operate/rs/networking/cluster-dns">}}), add the new node’s IP address to the DNS records.
+
 1. [Promote the first new node]({{<relref "/operate/rs/clusters/change-node-role/#promote-secondary-node">}}) to become the primary node.
 
 1. Verify node health:

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

@@ -80,6 +80,8 @@ To upgrade the cluster's operating system, use one of the following rolling upgr
 
 1. [Add the new node]({{< relref "/operate/rs/clusters/add-node" >}})  to the cluster.
 
+1. If the [cluster uses DNS]({{<relref "/operate/rs/networking/cluster-dns">}}), add the new node’s IP address to the DNS records.
+
 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. 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.
@@ -108,6 +110,8 @@ To upgrade the cluster's operating system, use one of the following rolling upgr
     rladmin cluster join nodes <cluster_member_ip_address> username <username> password <password> replace_node <node_id>
     ```
 
+1. If the [cluster uses DNS]({{<relref "/operate/rs/networking/cluster-dns">}}), add the new node’s IP address to the DNS records.
+
 1. Verify node health:
 
     1. Run `rlcheck` on all nodes: