Add gotcha to RDoc of collection_check_boxes

[ci skip]

actionview/lib/action_view/helpers/form_options_helper.rb

@@ -707,6 +707,27 @@ def collection_radio_buttons(object, method, collection, value_method, text_meth
       #   collection_check_boxes(:post, :author_ids, Author.all, :id, :name_with_initial) do |b|
       #      b.label(:"data-value" => b.value) { b.check_box + b.text }
       #   end
+      #
+      # ==== Gotcha
+      #
+      # When no selection is made for a collection of checkboxes most
+      # web browsers will not send any value.
+      #
+      # For example, if we have a +User+ model with +category_ids+ field and we
+      # have the following code in our update action:
+      #
+      #   @user.update(params[:user])
+      #
+      # If no +category_ids+ are selected then we can safely assume this field
+      # will not be updated.
+      #
+      # This is possible thanks to a hidden field generated by the helper method
+      # for every collection of checkboxes.
+      # This hidden field is given the same field name as the checkboxes with a
+      # blank value.
+      #
+      # In the rare case you don't want this hidden field, you can pass the
+      # <tt>include_hidden: false</tt> option to the helper method.
       def collection_check_boxes(object, method, collection, value_method, text_method, options = {}, html_options = {}, &block)
         Tags::CollectionCheckBoxes.new(object, method, self, collection, value_method, text_method, options, html_options).render(&block)
       end