Merge branch 'expiry' into 'master'

Clarify artifact expiry

## What does this MR do?

Clarifies documentation about artifact expiry.

## Are there points in the code the reviewer needs to double check?

## Why was this MR needed?

## What are the relevant issue numbers?

## Screenshots (if relevant)


See merge request !4831

doc/ci/yaml/README.md

@@ -726,8 +726,8 @@ time they are uploaded and stored on GitLab.
 You can use the **Keep** button on the build page to override expiration and
 keep artifacts forever.
 
-By default, artifacts are deleted hourly (via a cron job), but they are not
-accessible after expiry.
+After expiry, artifacts are actually deleted hourly by default (via a cron job),
+but they are not accessible after expiry.
 
 The value of `expire_in` is an elapsed time. Examples of parseable values:
 - '3 mins 4 sec'