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. This commit clarifies that only master-eligible nodes
are now involved with discovery.

Author: DaveCTurner

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