Remove/improve unused variables and weird doc comments

Signed-off-by: Nbeorn7 <beorn@grafana.com>

documentation/prometheus-mixin/config.libsonnet

 {
   _config+:: {
-    // Selectors are inserted between {} in Prometheus queries.
+    // prometheusSelector is inserted as part of the label selector in
+    // PromQL queries to identify metrics collected from Prometheus
+    // servers.
     prometheusSelector: 'job="prometheus"',
-    alertmanagerSelector: 'job="alertmanager"',
 
     // prometheusName is inserted into annotations to name the Prometheus
     // instance affected by the alert.

documentation/prometheus-mixin/dashboards.libsonnet

@@ -6,8 +6,6 @@ local g = import 'grafana-builder/grafana.libsonnet';
       g.dashboard('Prometheus')
       .addMultiTemplate('job', 'prometheus_build_info', 'job')
       .addMultiTemplate('instance', 'prometheus_build_info', 'instance')
-      # Prometheus is quite commonly configured with honor_labels set to true;
-      # therefore job and instance is not the prometheus server in many queries!
       .addRow(
         g.row('Prometheus Stats')
         .addPanel(