add comment about ISO 8601 and PHP 🙈

Signed-off-by: Morris Jobke <hey@morrisjobke.de>

config/config.sample.php

@@ -716,7 +716,9 @@ $CONFIG = array(
 /**
  * This uses PHP.date formatting; see http://php.net/manual/en/function.date.php
  *
- * Defaults to ``Y-m-d\TH:i:sO`` (ISO8601) - see \DateTime::ISO8601
+ * Defaults to ``Y-m-d\TH:i:sP`` (ISO8601 but with colon between hours and
+ * minutes of the timezone) - see \DateTime::ISO8601
+ * (https://secure.php.net/manual/en/class.datetime.php#datetime.constants.iso8601)
  */
 'logdateformat' => 'F d, Y H:i:s',