Document removal of references to "moderate" flag in core.

Per patch at http://drupal.org/node/72343, modules should no longer refer to the node moderate flag. I added a child page to the module update handbook page for the 4.7 to 4.8 update which should cover the information, but feel free to re-word, embellish, ask me questions, etc. The actual content should probably be incorporated in the parent page and the child page deleted, but I'll leave those sorts of decisions to the documentation gurus.