Update documentation to reflect artifact retention/keep behavior

Problem to solve

Our current documentation on Keep artifacts from most recent successful jobs does not specify that once customers toggle "keep latest" to off, the latest artifacts will be marked to be kept until another successful pipeline is run.

Until we are able to correct / change this behavior, out documentation should be updated to reflect the current status. Otherwise, customers (reasonably) think once the "keep" setting is turned off / disabled, that associated artifacts will expire per the expire_in setting.

Proposal

Add a note to existing documentation; something along the lines of:

After disabling **Keep artifacts from most recent successful jobs**, the latest artifacts will continue to be stored until the pipeline is run successfully again.