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

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

config/config_test.go

@@ -200,7 +200,7 @@ receivers:
 func TestTimeIntervalHasName(t *testing.T) {
 	in := `
 time_intervals:
-- name: 
+- name:
   time_intervals:
   - times:
      - start_time: '09:00'

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

@@ -487,7 +487,7 @@ You can use this mode if you suspect there is an issue with fallback mode or UTF
 
 You can use `amtool` to validate that an Alertmanager configuration file is compatible with UTF-8 strict mode before enabling it in Alertmanager server. You do not need a running Alertmanager server to do this.
 
-Just like Alertmanager server, `amtool` will log a warning if the configuration is incompatible or contains disagreement: 
+Just like Alertmanager server, `amtool` will log a warning if the configuration is incompatible or contains disagreement:
 
 ```
 amtool check-config config.yml
@@ -503,7 +503,7 @@ Found:
  - 0 templates
 ```
 
-You will know if a configuration is compatible with UTF-8 strict mode when no warnings are logged in `amtool`: 
+You will know if a configuration is compatible with UTF-8 strict mode when no warnings are logged in `amtool`:
 
 ```
 amtool check-config config.yml
@@ -644,7 +644,7 @@ Here are some more examples:
     ```
 
    As shown below, in the short-form, it's better to use double quotes to avoid problems with special characters like commas:
-   
+
    ```yaml
    matchers: [ "foo = \"bar,baz\"", "dings != bums" ]
    ```
@@ -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.
@@ -1458,6 +1459,6 @@ room_id: <string>
 # Message template.
 [ message: <tmpl_string> default = '{{ template "webex.default.message" .}}' ]
 
-# The HTTP client's configuration. You must use this configuration to supply the bot token as part of the HTTP `Authorization` header. 
+# The HTTP client's configuration. You must use this configuration to supply the bot token as part of the HTTP `Authorization` header.
 [ http_config: <http_config> | default = global.http_config ]
 ```