Clarify that discovery ignores master-ineligibles (#44835)

The changes in #32006 mean that the discovery process can no longer use
master-ineligible nodes as a stepping-stone between master-eligible nodes.
This was normally an indication of a strange and possibly-fragile configuration
and was not recommended, but this commit adds a note to the breaking changes
docs to note that this kind of configuration is more obviously broken in recent
versions.

Author: DaveCTurner

Diff for: docs/reference/migration/migrate_7_0/discovery.asciidoc

@@ -70,3 +70,17 @@ pings, each of which times out after 10 seconds. Thus a node that is
 unresponsive for longer than 30 seconds is liable to be removed from the
 cluster. Previously the default timeout for each ping was 30 seconds, so that
 an unresponsive node might be kept in the cluster for over 90 seconds.
+
+[float]
+==== Master-ineligible nodes are ignored by discovery
+
+In earlier versions it was possible to use master-ineligible nodes during the
+discovery process, either as seed nodes or to transfer discovery gossip
+indirectly between the master-eligible nodes. Clusters that relied on
+master-ineligible nodes like this were fragile and unable to automatically
+recover from some kinds of failure. Discovery now involves only the
+master-eligible nodes in the cluster so that it is not possible to rely on
+master-ineligible nodes like this. You should configure
+<<modules-discovery-hosts-providers,`discovery.seed_hosts` or another seed
+hosts provider>> to provide the addresses of all the master-eligible nodes in
+your cluster.

Diff for: docs/reference/modules/discovery/discovery.asciidoc

@@ -11,10 +11,10 @@ This process starts with a list of _seed_ addresses from one or more
 of any master-eligible nodes that were in the last-known cluster. The process
 operates in two phases: First, each node probes the seed addresses by
 connecting to each address and attempting to identify the node to which it is
-connected. Secondly it shares with the remote node a list of all of its known
-master-eligible peers and the remote node responds with _its_ peers in turn.
-The node then probes all the new nodes that it just discovered, requests their
-peers, and so on.
+connected and to verify that it is master-eligible. Secondly, if successful, it
+shares with the remote node a list of all of its known master-eligible peers
+and the remote node responds with _its_ peers in turn. The node then probes all
+the new nodes that it just discovered, requests their peers, and so on.
 
 If the node is not master-eligible then it continues this discovery process
 until it has discovered an elected master node. If no elected master is