Update documentation for Health Check Page to include an unhealthy example

https://docs.gitlab.com/ee/user/admin_area/monitoring/health_check.html

The page should include an example where the something is not healthy.

In my case, I am monitoring our instance for any issues. I am using Blackbox exporter and I can fail the probe if I match the regex, so knowing exactly what words are used to show status would be extremely beneficial.