[Transform] Updating frequency should apply after next checkpoint

[sophiec20]

Found in 8.2.0-SNAPSHOT

Documentation says
All updated properties except description do not take effect until after the transform starts the next checkpoint. This is so there is data consistency in each checkpoint.

However in practice, updating frequency does not "seem" to apply until the transform is stopped and started.

To reproduce:

1. Created and started a continuous transform on a dataset (timestamps ranging between now-10d to now+10d), with frequency: 30s
2. Closed the source index POST source-index/_close
3. Observed timestamps of warning notifications, 30s apart
4. Opened source index POST source-index/_open
5. Updated transform to set frequency: 10s
6. Inserted current timestamp docs into source index (to force the checkpoint)
7. Observed notification message confirming checkpoint occurred
8. Closed the source index
9. Observed timestamps of warning notifications -- 30s apart, instead of expected 10s apart
10. Open the source index
11. Stop and restart the transform
12. Close the source index
13. Observed timestamps of warning notifications -- now at 10s apart

Despite updating frequency: 10s beforehand, the notification messages still show that the action is occurring every 30s after a checkpoint. After stopping and restarting the transform, then the notification messages occur every 10s.

Either
1/ the docs require a clarification, or
2/ the test isn't valid, or
3/ there is a bug in the way frequency is being applied

[elasticmachine]

Pinging @elastic/ml-core (Team:ML)