Add sliding window text

[osirisinferi]

On the Community (https://community.letsencrypt.org/t/failed-validation-limit-how-long-should-i-wait/210342) it seems like the failed authorization documentation page didn't explain how long a user should wait once the rate limit was hit.

The general rate limit page mentions the sliding window as means of determining the rate limit. I thought maybe mentioning it on the failed authz page might help users understand it better :slightly_smiling_face: