write D7 version of the FormAPI guide

This has been recently moved from the API site to the handbooks: http://drupal.org/node/751826

I'd say it's a key component of working with the FormAPI, and a page for the D7 version of this needs to be created.

The first major change I've noticed is that the form callback has different parameters:

D6:

function test_form($form_state) {

D7:

/**
 * Form builder; User administration page.
 *
 * @ingroup forms
 * @see user_admin_account_validate()
 * @see user_admin_account_submit()
 */
function user_admin_account() {

No parameters on examples in core. And yet func_get_args shows stuff.