This PR improves the SQS broker documentation to clarify a common pitfall while setting up Celery with SQS.

Problem: The current documentation shows how to URL-encode AWS credentials for the broker URL using
safequote(), but doesn't clarify that these encoded credentials should NOT be used in the predefined_queues
configuration. Doing so will throw an error that looks something like this

Unrecoverable error: ClientError("An error occurred (SignatureDoesNotMatch) when calling the GetQueueAttributes
operation: The request signature we calculated does not match the signature you provided. 
Check your AWS Secret Access Key and signing method. Consult the service documentation for details.

This docs change should make the SQS setup more reliable and intuitive for users working with predefined queues.