add pointer to which IP address ranges are blocked by SSRF protection

app/mailers/user_mailer.rb

@@ -33,7 +33,7 @@ class UserMailer < ApplicationMailer
 
   helper_method :message_url
 
-  def test_mail(user, delivery_method_options:)
+  def test_mail(user, delivery_method_options: {})
     @welcome_url = url_for(controller: "/homescreen")
 
     open_project_headers "Type" => "Test"

config/constants/settings/definition.rb

@@ -1181,10 +1181,12 @@ module Settings
       },
       ssrf_protection_ip_allowlist: {
         description: "
-          Connections to certain IP addresses (such as private ranges) are blocked to prevent SSRF attacks.
+          Connections to certain IP addresses (such as private ranges, c.f. [1]) are blocked to prevent SSRF attacks.
           Use this setting to explicitly allow given IP addresses which would otherwise be blocked.
           Takes a comma or space separated list of IPv4 and IPv6 addresses (including masks for ranges),
           e.g. `192.168.255.255/16`.
+
+          [1] https://github.com/arkadiyt/ssrf_filter/blob/main/lib/ssrf_filter/ssrf_filter.rb#L28-L58
         ".squish,
         format: :string,
         default: "",