chore(configuration): add reference to Prometheus endpoint in metrics…

… documentation

The cross reference makes it easier to pair the web server and the
metrics configuration elements.

Signed-off-by: Samuel Gaist <samuel.gaist@idiap.ch>

falco.yaml

@@ -970,6 +970,9 @@ syscall_event_drops:
 # beneficial for exploring the data schema and ensuring that fields with empty
 # values are included in the output.
 #
+# If metrics are enabled, the web server can be configured to activate the
+# corresponding Prometheus endpoint using webserver.prometheus_metrics_enabled.
+#
 # todo: prometheus export option
 # todo: syscall_counters_enabled option
 metrics: