[documentation] FAPI/#ahah docs

[Peter Wolanin]

Recently, Augustin (beginner) has asked some pointed questions about
the sorry state of AJAX/JS documentation for D6 and started clening up
the handbook.

When I looked at what had been added to the developer docs, it was
clear that it was totally out of date with the current forms.inc &
ahah.js implementation.

So, I spent a number of hours working on this, and Augustin reviewed
the changes and made more changes and corrections.  The catch - I'm
really not that familiar with JS or the #ahah form element.

So, please take a look at the current version if you are familiar with them:

http://api.drupal.org/api/file/developer/topics/forms_api_reference.html/6#ahah

If you think the changes are correct and satisfactory, mark this issue fixed:

http://drupal.org/node/154398

If the changes are not correct - then fix it please!

-Peter