Clarify behavior of the status field output when a scheduled backup job fails

[livlobo]

Liv Lobo (livlobo) commented:

Show schedules docs explain this as -- status = Displays last-known errors or messages about the backup schedule

Maybe we can be more explicit about this field being cleared on retry of a scheduled job.

Slack threads here summarize it:
https://cockroachlabs.slack.com/archives/C2C5FKPPB/p1631887692197300
https://cockroachlabs.slack.com/archives/CHVV403F0/p1631880896286500

Jira Issue: DOC-1271

[chaneth8]

I would like to work on this issue.