Document *all* the options accepted by form_for

The [current documentation of form_for](http://api.rubyonrails.org/classes/ActionView/Helpers/FormHelper.html#method-i-form_for) is incomplete:

> The rightmost argument to form_for is an optional hash of options -
>   :url - The URL the form is to be submitted to. This may be represented in the same way as values passed to url_for or link_to. So for example you may use a named route directly. When the model is represented by a string or symbol, as in the example above, if the :url option is not specified, by default the form will be sent back to the current url (We will describe below an alternative resource-oriented usage of form_for in which the URL does not need to be specified explicitly).
>   :namespace - A namespace for your form to ensure uniqueness of id attributes on form elements. The namespace attribute will be prefixed with underscore on the generated HTML id.
>   :html - Optional HTML attributes for the form tag.

This commit completes the documentation by specifying that:

* `:authenticity_token` is also a valid option (as [documented here](https://github.com/rails/rails/blob/b0f44c9ff0be84ebc42760029adeeb9dd954655f/actionview/lib/action_view/helpers/form_helper.rb#L396))
* `:method` is also a valid option (as [tested here](https://github.com/rails/rails/blob/b0f44c9ff0be84ebc42760029adeeb9dd954655f/actionview/test/template/form_helper_test.rb#L1819))
* `:remote` is also a valid option (as [tested here](https://github.com/rails/rails/blob/b0f44c9ff0be84ebc42760029adeeb9dd954655f/actionview/test/template/form_helper_test.rb#L1850))
* `html: {multipart: true}` does not simply add an HTML attribute but generates a multipart form as [tested here](https://github.com/rails/rails/blob/b0f44c9ff0be84ebc42760029adeeb9dd954655f/actionview/test/template/form_helper_test.rb#L3202)
* `html: {enforce_utf8: false}` does not simply add an HTML attribute but skips the UTF8 enforcer as tested in the only test added by this commit