Document getMinTransportVersion including exceptions (elastic#124192)

The cluster's min transport version has some useful properties but also
some subtle exceptions that are today not documented in the code. This
commit adds some Javadocs in this area.

Author: DaveCTurner

server/src/main/java/org/elasticsearch/cluster/ClusterState.java

@@ -322,6 +322,28 @@ public boolean hasMixedSystemIndexVersions() {
             .anyMatch(e -> e.systemIndexMappingsVersion().equals(minVersions.systemIndexMappingsVersion()) == false);
     }
 
+    /**
+     * @return the minimum {@link TransportVersion} that will be used for all future intra-cluster node-to-node communications. This value
+     *         only ever increases, so if {@code v.onOrAfter(cs.getMinTransportVersion())} is true once then it will remain true in the
+     *         future.
+     * <p>
+     * There are some subtle exceptions:
+     * <ul>
+     * <li>The "only ever increases" property is handled by the master node using the in-memory (ephemeral) part of the
+     *     {@link ClusterState} only, so in theory a full restart of a mixed-version cluster may lose that state and allow some nodes to see
+     *     this value decrease. For this to happen in practice requires some fairly unlucky timing during the initial master election. We
+     *     tell users not to do this: if something breaks during a rolling upgrade then they should upgrade all remaining nodes to continue.
+     *     But we do not enforce it.
+     * <li>The "used for all node-to-node communications" is false in a disordered upgrade (an upgrade to a semantically-newer but
+     *     chronologically-older version) because for each connection between such nodes we will use {@link
+     *     TransportVersion#bestKnownVersion} to pick a transport version which is known by both endpoints. We tell users not to do
+     *     disordered upgrades too, but do not enforce it.
+     * </ul>
+     * <p>
+     * Note also that node-to-node communications which are not <i>intra-cluster</i> (i.e. they are not between nodes in the same cluster)
+     * may sometimes use an earlier {@link TransportVersion} than this value. This includes remote-cluster communication, and communication
+     * with nodes that are just starting up or otherwise are attempting to join this cluster.
+     */
     public TransportVersion getMinTransportVersion() {
         return this.minVersions.transportVersion();
     }