Clarify futher the order for a rolling upgrade (#52964)

Expands the "master-ineligible then master-eligible" sentence into a list and
specifies that within these subsets the order doesn't matter.

Author: DaveCTurner

docs/reference/upgrade/rolling_upgrade.asciidoc

@@ -7,11 +7,27 @@ a time so upgrading does not interrupt service. Running multiple versions of
 not supported, as shards cannot be replicated from upgraded nodes to nodes
 running the older version.
 
-It is best to upgrade the master-eligible nodes in your cluster after all of
-the other nodes. Once you have started to upgrade the master-eligible nodes
-they may form a cluster that nodes of older versions cannot join. If you
-upgrade the master-eligible nodes last then all the other nodes will not be
-running an older version and so they will be able to join the cluster.
+We strongly recommend that when you upgrade you divide your cluster's nodes
+into the following two groups and upgrade the groups in this order:
+
+. Nodes that are not <<master-node,master-eligible>>. You can retrieve a list
+of these nodes with `GET /_nodes/_all,master:false` or by finding all the nodes
+configured with `node.master: false`.
+
+. Master-eligible nodes, which are the remaining nodes. You can retrieve a list
+of these nodes with `GET /_nodes/master:true`.
+
+You may upgrade the nodes within each of these groups in any order.
+
+Upgrading the nodes in this order ensures that the master-ineligible nodes are
+always running a version at least as new as the master-eligible nodes. Newer
+nodes can always join a cluster with an older master, but older nodes cannot
+always join a cluster with a newer master. By upgrading the master-eligible
+nodes last you ensure that all the master-ineligible nodes will be able to join
+the cluster whether the master-eligible nodes have been upgraded or not. If you
+upgrade any master-eligible nodes before the master-ineligible nodes then there
+is a risk that the older nodes will leave the cluster and will not be able to
+rejoin until they have been upgraded.
 
 Rolling upgrades are supported: