Add sliding window text (#1613)

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 🙂

content/en/docs/failed-validation-limit.md

@@ -1,15 +1,16 @@
 ---
 title: Failed Validation Limit
 slug: failed-validation-limit
-lastmod: 2024-06-21
+lastmod: 2024-07-04
 show_lastmod: false
 ---
 
 
 # Description
 All issuance requests are subject to a *Failed Validation* limit of 5 failures
-per account, per hostname, per hour. You should receive the following error
-message from your ACME client when you’ve exceeded the Failed Validation limit:
+per account, per hostname, per hour (using a sliding window). You should
+receive the following error message from your ACME client when you’ve exceeded
+the Failed Validation limit:
 
 ```
 too many failed authorizations recently: see https://letsencrypt.org/docs/failed-validation-limit/
@@ -54,4 +55,4 @@ on our community forum](https://community.letsencrypt.org/c/help/13).
 
 # Requesting an Override
 
-Overrides are **not** available for the Failed Validation limit.
+Overrides are **not** available for the Failed Validation limit.