MDL-15544 add danger warning for email self-registration; backported from HEAD

lang/en_utf8/auth.php

@@ -330,7 +330,7 @@ $string['nopasswordchangeforced'] ='You cannot proceed without changing your pas
 $string['passwordhandling'] = 'Password field handling';
 $string['plaintext'] = 'Plain text';
 $string['selfregistration'] = 'Self registration';
-$string['selfregistration_help'] = 'Choose which auth plugin will handle user self-registration.';
+$string['selfregistration_help'] = 'If an authentication plugin, such as email-based self-registration, is selected, then it enables potential users to register themselves and create accounts. This results in the possibility of spammers creating accounts in order to use forum posts, blog entries etc. for spam. To avoid this risk, self-registration should be disabled or limited by <em>Allowed email domains</em> setting.';
 $string['sha1'] = 'SHA-1 hash';
 $string['showguestlogin'] = 'You can hide or show the guest login button on the login page.';
 $string['stdchangepassword'] = 'Use standard Change Password Page';