Updated docs for MessageCleanupIntervalSecs env var to clarify TTL vs…

… MessageCleanupIntervalSecs behaviour. (#7314)

Updated docs for MessageCleanupIntervalSecs env var to clarify TTL vs MessageCleanupIntervalSecs behaviour.

## Azure IoT Edge PR checklist:

doc/EnvironmentVariables.md

@@ -77,7 +77,7 @@ You can set Edge Agent and Edge Hub environment variables in the Azure portal. I
 | MaxConnectedClients                       | Maximum number of downstream clients allowed to connect                                                                                            | int32                                                                 | 101 (100 clients + 1 EdgeHub) |
 | MaxUpstreamBatchSize                      | Max number of messages to concurrently send upstream                                                                                               | int32                                                                 | 10                            |
 | MessageAckTimeoutSecs                     | Time span to wait for sending a message downstream to a leaf device                                                                                | int32                                                                 | 30                            |
-| MessageCleanupIntervalSecs                | Time interval for task that cleans up messages in edgeHub's store                                                                                  | int32                                                                 | 1800                          |
+| MessageCleanupIntervalSecs                | This setting defines the time interval for a task that cleans up messages in EdgeHub's store. Note that messages are only removed from the store when this task runs. If you use a [Time To Live](https://github.com/Azure/iotedge/blob/main/doc/Route_priority_and_TTL.md) that is shorter than the default cleanup interval, please adjust the cleanup interval accordingly to ensure timely message removal.                                                                                  | int32                                                                 | 1800                          |
 | Metrics__Listener__Host                   | Hostname of the metrics listener, used to construct the metrics listener URL                                                                       | string                                                                | *                             |
 | Metrics__Listener__MetricsEnabled         | Whether to enable metrics listener                                                                                                                 | bool                                                                  | true                          |
 | Metrics__Listener__MetricsHistogramMaxAge | Time interval for the metrics histogram                                                                                                            | TimeSpan string                                                       | 00:01:00 (1 hour)             |