[development] Minor changes on form api docs

[Robert Garrigós Castro]

I have made some minor changes on the 
http://drupaldocs.org/api/head/file/contributions/docs/developer/topics/forms_api_reference.html 
page because of missing property #submit (former #execute) and a 
deprecated note on #extra property (now #attributes). I discussed this 
with chx and applied the changes. That was 4 hours ago. I don't know 
whether some one has to have it a look in order to these changes be 
available to the web or not. If that's the case, this is a reminder. 
Sorry if it's just a matter of time.

Robert Garrigos