Fixes according to the code review

Signed-off-by: m.nabokikh <maksim.nabokikh@flant.com>

docs/configuration/configuration.md

@@ -3415,6 +3415,10 @@ static_configs:
 # List of Alertmanager relabel configurations.
 relabel_configs:
   [ - <relabel_config> ... ]
+
+# List of alert relabel configurations.
+alert_relabel_configs:
+  [ - <relabel_config> ... ]
 ```
 
 ### `<remote_write>`

notifier/notifier.go

@@ -383,13 +383,11 @@ func relabelAlerts(relabelConfigs []*relabel.Config, externalLabels labels.Label
 
 	for _, a := range alerts {
 		lb.Reset(a.Labels)
-		if externalLabels.Len() > 0 {
 		externalLabels.Range(func(l labels.Label) {
 			if a.Labels.Get(l.Name) == "" {
 				lb.Set(l.Name, l.Value)
 			}
 		})
-		}
 
 		keep := relabel.ProcessBuilder(lb, relabelConfigs...)
 		if !keep {