Manual talk:$wgDebugLogFile

The documentation should say how to use the default webserver log file (I mean the standard error_log() function in PHP).

Its really a PHP thing. You can easily write simple php statements in LocalSettings.php that do this like:

error_log("LDAPStack [OK] - JSON file found")

While that is not an error message per-se its just text end of day. However all of these will show up classified as 'errors' in php-fpm log and http logs as it is an 'error_log' facility.

-HTH

Uh? I mean, how to tell MediaWiki to just rely on error_log() globally for its error messages, like whatever PHP application does as default.

Probably by doing this:

$wgDebugLogGroups = array(
	'exception' => ini_get('error_log'),
	'error' => ini_get('error_log'),
);

Nice workaround but honestly I don't know if manually writing in the same log file concurrently used by the webserver it's a good idea.