Reinstate recommendation for ≥ 3 master-eligible nodes. (#27204)

In the docs for 1.7 ([doc][doc-1.7], [src][src-1.7]) there was a recommendation
for at least 3 master-eligible nodes "in critical clusters" but this was lost
when that page was updated in 2.0 ([doc][doc-2.0], [src][src-2.0]). I'd like to
reinstate this.

[doc-1.7]: https://www.elastic.co/guide/en/elasticsearch/reference/1.7/modules-node.html
[src-1.7]: https://github.com/elastic/elasticsearch/blob/2cbaccb2f2a495923bc64447fe3396e0fc58b3d3/docs/reference/modules/node.asciidoc
[doc-2.0]: https://www.elastic.co/guide/en/elasticsearch/reference/2.0/modules-node.html#split-brain
[src-2.0]: https://github.com/elastic/elasticsearch/blob/4799009ad7ea8f885b6aedc6f62ad61d69e7a40d/docs/reference/modules/node.asciidoc

docs/reference/modules/node.asciidoc

@@ -156,6 +156,13 @@ discovery.zen.minimum_master_nodes: 2 <1>
 ----------------------------
 <1> Defaults to `1`.
 
+To be able to remain available when one of the master-eligible nodes fails,
+clusters should have at least three master-eligible nodes, with
+`minimum_master_nodes` set accordingly. A <<rolling-upgrades,rolling upgrade>>,
+performed without any downtime, also requires at least three master-eligible
+nodes to avoid the possibility of data loss if a network split occurs while the
+upgrade is in progress.
+
 This setting can also be changed dynamically on a live cluster with the
 <<cluster-update-settings,cluster update settings API>>: