Add message to warn the user if a form has too many fields.

Hi,
Here is a pull request for http://sourceforge.net/p/phpmyadmin/bugs/4319/
As said in http://sourceforge.net/p/phpmyadmin/bugs/4319/#3d94 , this is not a pMA issue, so I added a warning, not a fix.
Is it ok?
Could you also check the english sentence please?
Thanks.