Create Alertmanager silence form does not explain the new "Negative matcher" option

The Alertmanager silence create / edit form got a new "Negative matcher" option in 4.12 (see https://issues.redhat.com/browse/OCPBUGSM-47734). However, there is nothing to explain what this option means and it will likely not be obvious from the label alone unless you are already quite familiar with Alertmanager.

After discussion with the docs team, it was decided that adding some explanation in context in the UI would be much better than adding an explanation to the documentation. 

Always

1. Go to Admin perspective
2. Go to Observe > Alerting > Silences page
3. Click on the Create button ("Negative matcher" option is shown with no explanation)