docs: better descibe `email_config.to` fromat (#3760)

Signed-off-by: Christoph Maser <christoph.maser+github@gmail.com>

config/notifiers_test.go

@@ -15,6 +15,8 @@ package config
 
 import (
 	"errors"
+	"net/mail"
+	"reflect"
 	"strings"
 	"testing"
 
@@ -60,6 +62,47 @@ headers:
 	}
 }
 
+func TestEmailToAllowsMultipleAdresses(t *testing.T) {
+	in := `
+to: 'a@example.com, ,b@example.com,c@example.com'
+`
+	var cfg EmailConfig
+	err := yaml.UnmarshalStrict([]byte(in), &cfg)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	expected := []*mail.Address{
+		{Address: "a@example.com"},
+		{Address: "b@example.com"},
+		{Address: "c@example.com"},
+	}
+
+	res, err := mail.ParseAddressList(cfg.To)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if !reflect.DeepEqual(res, expected) {
+		t.Fatalf("expected %v, got %v", expected, res)
+	}
+}
+
+func TestEmailDisallowMalformed(t *testing.T) {
+	in := `
+to: 'a@'
+`
+	var cfg EmailConfig
+	err := yaml.UnmarshalStrict([]byte(in), &cfg)
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, err = mail.ParseAddressList(cfg.To)
+	if err == nil {
+		t.Fatalf("no error returned, expected:\n%v", "mail: no angle-addr")
+	}
+}
+
 func TestPagerdutyTestRoutingKey(t *testing.T) {
 	t.Run("error if no routing key or key file", func(t *testing.T) {
 		in := `

docs/configuration.md

@@ -867,6 +867,7 @@ webhook_url_file: <filepath>
 [ send_resolved: <boolean> | default = false ]
 
 # The email address to send notifications to.
+# Allows a comma separated list of rfc5322 compliant email addresses.
 to: <tmpl_string>
 
 # The sender's address.