Added short doc for component clustering and added upgrade for mqtt node

application/src/main/data/upgrade/3.4.4/schema_update.sql

@@ -615,7 +615,7 @@ $$;
 
 ALTER TABLE rule_node ADD COLUMN IF NOT EXISTS singleton_mode bool DEFAULT false;
 
-UPDATE rule_node SET singleton_mode = true WHERE type = 'org.thingsboard.rule.engine.mqtt.azure.TbAzureIotHubNode';
+UPDATE rule_node SET singleton_mode = true WHERE type IN ('org.thingsboard.rule.engine.mqtt.azure.TbAzureIotHubNode', 'org.thingsboard.rule.engine.mqtt.TbMqttNode');
 
 ALTER TABLE component_descriptor ADD COLUMN IF NOT EXISTS clustering_mode varchar(255) DEFAULT 'ENABLED';
 

common/data/src/main/java/org/thingsboard/server/common/data/plugin/ComponentClusteringMode.java

@@ -15,6 +15,13 @@
  */
 package org.thingsboard.server.common.data.plugin;
 
+/**
+ * The main idea to use this - it's adding the ability to start rule nodes in singleton mode in cluster setup
+ * (singleton rule node will start in only one Rule Engine instance)
+ * USER_PREFERENCE - user has ability to configure clustering mode (enable/disable singleton mode in rule node config)
+ * ENABLE - user doesn't have ability to configure clustering mode (singleton mode is always FALSE in rule node config)
+ * SINGLETON - user doesn't have ability to configure clustering mode (singleton mode is always TRUE in rule node config)
+ */
 public enum ComponentClusteringMode {
     USER_PREFERENCE,
     ENABLED,