Note about using "with" keyword with form_theme to pass an array

[caponica]

Add a note to the form theming docs to say that you can pass an array if you use the "with" keyword first. Trying to pass an array without "with" causes an error but luckily I found a github thread that mentioned the magic "with":

symfony/symfony#3576 (comment)

Would be better if a note about this could be added to the docs instead.