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))

[ci skip]

[ci skip]

89ff1f82 and 1de258e6 removed from the HTML generated by the form helpers
the <div> that was wrapping the field elements inside the <form>.

This commit updates the documentation of the methods to reflect the
two commits above.

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This allows rails code to be more confdent when asking for a model name, instead of having to ask for the class.

Rails core discussion here: https://groups.google.com/forum/#!topic/rubyonrails-core/ThSaXw9y1F8

For some reason, if the hash is empty, we choose "optimized" routes.  I
am not sure how or why optimized routes differ from regular routes.
But, if we make sure the hash is empty, we get a nice speed improvement.

The use of `display:inline` with the content_tag call in the
extra_tags_for_form method potentially causes display issues with some
browsers, namely Internet Explorer. IE's behaviour of not collapsing
the line height on divs with ostensibly no content means that the
automatically added div containing the hidden authenticity_token, utf8
and _method form input tags may interfere with other visible form
elements in certain circumstances. The use of `display:none` rather
than `display:inline` fixes this problem.

Fixes #6403

:as option should not overwrite :namespace option when
generating html id attribute of the form element. id should be prefixed
by specified namespace even if :as option is present

Add test case showing the issue and code fixing it

The option to disable including a hidden ID field on a `fields_for` nested association was incorrectly documented as `hidden_field_id` instead of `include_id`

clarify issue #10115. Also made the field_helpers an explicit list of
methods.

there are just the same description about accepts_nested_attributes [ci skip]

By checking for object.persisted? first, we avoid the hash lookups for
new objects.

When dealing with nested forms, Rails automatically generates a hidden
field with the id value of the current object being generated by
fields_for. This logic was inside the method that's available from the
template object, but we just need it when really dealing with nested
attributes, so moving the code to here makes more sense.

Just use it internally from fields_for until we come up with a better solution.

This reverts commit ba2d867b.

Reason:
https://github.com/lifo/docrails/commit/ba2d867b2961b5ef72aad353c2d647b04654cbe3#commitcomment-2276670

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