Add unknwon status to crons monitor details docs

Author: gaprl

docs/product/crons/monitor-details.mdx

@@ -23,6 +23,7 @@ Sentry's Cron Monitoring service can notify you and store a timeline of the foll
   - An invalid request format.
 - **Failed check-in:** The job has reported its execution as unsuccessful.
 - **Successful check-in:** The job has reported its execution as successful.
+- **Unknown check-in:** In rare occasions, it is possible that Sentry is unable to identify a check-in status. This can happen during outages or maintenance periods.
 
 To see information about your cron monitor, select "Crons" from the sidebar menu and click on your cron monitor's name.