How do I specify the debug_mail_directory for the devel maillog? In settings.local.php, I have set my local dev environment to use the Devel module's mail logging function:

 $config['system.mail']['interface']['default'] = 'devel_mail_log'; 
 $config['devel']['settings']['debug_mail_directory'] = "???";


The DevelMailLog documentation says that:


  Filename pattern controlled by 'debug_mail_file_format' config setting.


What do I put for the filename pattern?  I have tried several patterns:


public://devel_mail
private://emails
sites/default/files/devel


But none of these work.  I checked watchdog, but apparently devel mail doesn't leave any logs.

NOTE 1: There is a note in the docs that config settings are broken, but the linked issue is already fixed.

NOTE 2: By default, when I enable devel and export the config, the value for debug_mail_directory is temporary://devel-mails, but I don't want to store them in the temporary directory; I want to store them in the Drupal files directory if possible.
You should use 

$config['devel.settings']['debug_mail_directory'] = 'private://mydirectory';
$config['devel.settings']['debug_mail_file_format'] = 'mypattern-%to-%subject-%datetime.mail.txt';