Document required path variable in settings.php

To use the "langexp" command in connection with the customtranslations module, one needs to set the path the translations are exported to in settings.php
The setting is to be defined like this

$settings['custom_translations_directory'] = [path-relative-to-drupal-root]

So if you use a composer template which puts the drupal root one level up from the git root, your path might be '../custom-translations' so it creates a directory in the git root path.

This is not documented anywhere. It should be in the readme of one of the two modules, not sure which.
Today I wanted to set the path but could for the life of me find not any documentation.
Until I remembered trying to run the drush command, then you get helped...

If this never happens, I least I can find this issue, which helps as a kind of stealth documentation }:-]<

Filing as a documentation bug