The meaning of update.php pending updates I have replaced Drupal 7 core files from version 7.10 to 7.12. Now I am running update.php and it displays 3 pending updates, e.g.: 


  menu module
  7003 - Add missing custom menus to active menus list


How do I get more information what this means exactly? I checked the site with release notes for Drupal 7.12, but there is no such string (7003) in that list.
It's part of menu_update_7003(). The comments in that function say it all really:

Make sure all custom menus are present in the active menus variable so that their items may appear in the active trail.

Usually just searching Google for the exact message in quotes (e.g. "Add missing custom menus to active menus list") will bring up the page that explains why it's been done (that's what I did to find this out). You can most likely do the same for the other two updates and find out exactly what they're doing.