docs(config): inform about log.backtrace

Signed-off-by: Arthur Schiwon <blizzz@arthur-schiwon.de>

config/config.sample.php

@@ -998,6 +998,16 @@ $CONFIG = [
 	'apps' => ['files'],
 ],
 
+/**
+ * Enables logging a backtrace with each log line. Normally, only Exceptions
+ * are carrying backtrace information which are logged automatically. This
+ * switch turns them on for any log message. Enabling this option will lead
+ * to increased log data size.
+ *
+ * Defaults to ``false``.
+ */
+'log.backtrace' => false,
+
 /**
  * This uses PHP.date formatting; see https://www.php.net/manual/en/function.date.php
  *