That is not a documentation error. It doesn’t make a difference if you set it between double or single quotes.
As far as I could understand from your other posting and the provided log file entries shown in it, you prepended the line "maintenance" => true, to the array like this:
<?php
"maintenance" => true,
$CONFIG = array (
That resulted in the error:
PHP Parse error: syntax error, unexpected token "=>" in /var/www/nextcloud/config/config.php on line 2
The configuration setting “maintenance” is always in the array, and when changing it manually, you only have to set the value from true to false (or vice versa) and the line must never be placed in front of the array, which according to the log file echo you must have accidentally tried once.
So it is not a documentation error but an interpretation error.