Documentation detail needed (hook system)

To prevent confusions (see for example the threads Understanding the hook system, and HOOK SYSTEM - does anyone understand it?), I believe a short explanation like the following -or similar- should be added to the introductory text on http://api.drupal.org/api/group/hooks :

"The hook codes included here are not part of the Drupal core, they are just models that you can modify. Only the hook functions implemented within modules are executed when running Drupal."