How to change the text in the revision log message?
Use the following code in a custom module or in your theme's template.php:

/**
 * Implements hook_form_FORM_ID_alter().
 */
function YOURMODULE_form_CONTENTTYPE_node_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  $form['revision_information']['log']['#description'] = t('Your new instructions.');
}


Note: replace YOURMODULE with your module/theme name and CONTENTTYPE with machine name of the content type you want to change.