[development] New docs for multipage forms in drupal.org handbook

A few months ago during the crazy dash to rewrite FormAPI, I wrote up a guide to making multistep forms and posted it to my blog. It's now been moved to the Drupal.org handbook at http://drupal.org/node/101707 where it can do more good. It's a bit wordy, and it includes some historical background for 4.7 that's probably unnecessary now, but to the best of my knowledge it's the only place that the nature of the #multistep flag is documented. :-) In the future I'd like to update the multistep example module in contrib to match this article, too. --Jeff Eaton