Add note about the polling cycle
Add note about the polling cycle

[`s3_wait>` article](https://docs.digdag.io/operators/s3_wait.html)
doesn't mention the polling cycle yet.
So, I'd like to mention it in our doc.

https://github.com/treasure-data/digdag/blob/42ea7cb71270957ea6424c2df88dd7c65d248789/digdag-standards/src/main/java/io/digdag/standards/operator/aws/S3WaitOperatorFactory.java#L47

treasure-data/digdag,treasure-data/digdag,treasure-data/digdag,treasure-data/digdag,treasure-data/digdag,treasure-data/digdag,treasure-data/digdag