Document how updates to job config files make it to the Prow cluster

[irbekrm]

As I understand the way this works is by using the config_updater plugin that watches the files in jobs/ and updates a job-config configmap in Prow's build cluster.
However, lately we've had weird issues when after i.e a new jobs are added or some file names have been changed, the correct job configs are not propagated to the cluster. See i.e cert-manager/csi-driver-spiffe#2 (comment) and the weird output of the update from this PR #591 which renamed a file.
We should:

1. investigate whether the config updater works as it should
2. document how it works