Added info about severity

docs/notification-routing/routing-by-severity.rst

@@ -22,4 +22,8 @@ Configure Slack to send high-severity alerts to one channel and all others to an
         scope:
         exclude: # excludes so you don't get the same alert in two channels
             - severity: HIGH
-              source: PROMETHEUS
+              source: PROMETHEUS
+
+
+**Important:** ``severity`` above refers to the Robusta severity for this alert. `Robusta maps Prometheus severities onto standardized levels.<https://docs.robusta.dev/master/setup-robusta/additional-settings.html#mapping-custom-alert-severity>`_. When specifying the severity, use the Robusta severity not the original Prometheus severity.
+