IoT: Add MQTT broker limitations (#1161)

content/en/user-guide/aws/iot/index.md

@@ -180,8 +180,15 @@ You can then subscribe or use topic rules on the follow topics:
 
 ## Topic Rules
 
-It is also possible to use advanced features like SQL queries for IoT topic rules.
+It is possible to use actions with SQL queries for IoT Topic Rules.
 
 For example, you can use the [`CreateTopicRule`](https://docs.aws.amazon.com/iot/latest/apireference/API_CreateTopicRule.html) operation to define a topic rule with a SQL query `SELECT * FROM 'my/topic' where attr=123` which will execute a trigger whenever a message with attribute `attr=123` is received on the MQTT topic `my/topic`.
 
 Supported triggers include Kinesis, Lambda, SQS, Firehose and DynamoDB v2.
+
+## Limitations
+
+LocalStack MQTT broker does not support multi-account/multi-region namespacing.
+Internally, the MQTT messages are not routed to the appropriate account ID/region even though the endpoint URL may suggest otherwise.
+All messages will be routed to the `000000000000` account and the `us-east-1` region.
+This prevents features such as topic rules from working properly when not using the this account ID or region.