[es/versionCheck] explain why we need to track per-server

src/core_plugins/elasticsearch/lib/check_es_version.js

@@ -9,8 +9,14 @@ import semver from 'semver';
 import isEsCompatibleWithKibana from './is_es_compatible_with_kibana';
 import SetupError from './setup_error';
 
-// tracks the node descriptions that get logged in warnings so
-// that we don't spam the log with the same message over and over
+/**
+ *  tracks the node descriptions that get logged in warnings so
+ *  that we don't spam the log with the same message over and over.
+ *
+ *  There are situations, like in testing or multi-tenancy, where
+ *  the server argument changes, so we must track the previous
+ *  node warnings per server
+ */
 const lastWarnedNodesForServer = new WeakMap();
 
 module.exports = function checkEsVersion(server, kibanaVersion) {