fix(messaging): Clarify ApproximateNumberOfMessagesNotVisible

Author: JulienVdG

serverless/messaging/reference-content/sqs-support.mdx

@@ -254,7 +254,7 @@ The Scaleway Queues API (currently named the SQS API) supports the following que
 | All                                   |   **Y**   |            |
 | ApproximateNumberOfMessages           |   **Y**   |            |
 | ApproximateNumberOfMessagesDelayed    |   **N**   |            |
-| ApproximateNumberOfMessagesNotVisible |   **Y**   |            |
+| ApproximateNumberOfMessagesNotVisible |   **Y**   | When a message has been delivered once, it will be counted here until it is deleted, even if its VisibilityTimeout expires. |
 | ContentBasedDeduplication             |   **Y**   |            |
 | CreatedTimestamp                      |   **Y**   |            |
 | DeduplicationScope                    |   **N**   |            |