mention strong parameters in complex forms section of the guides.

Closes #9921.

guides/source/form_helpers.md

@@ -906,7 +906,21 @@ If the associated object is already saved, `fields_for` autogenerates a hidden i
 
 ### The Controller
 
-You do not need to write any specific controller code to use nested attributes. Create and update records as you would with a simple form.
+As usual you need to
+[whitelist the parameters](action_controller_overview.html#strong-parameters) in
+the controller before you pass them to the model:
+
+```ruby
+def create
+  @person = Person.new(person_params)
+  # ...
+end
+
+private
+def person_params
+  params.require(:person).permit(:name, addresses_attributes: [:id, :kind, :street])
+end
+```
 
 ### Removing Objects
 
@@ -937,6 +951,16 @@ If the hash of attributes for an object contains the key `_destroy` with a value
 <% end %>
 ```
 
+Don't forget to update the whitelisted params in your controller to also include
+the `_destroy` field:
+
+```ruby
+def person_params
+  params.require(:person).
+    permit(:name, addresses_attributes: [:id, :kind, :street, :_destroy])
+end
+```
+
 ### Preventing Empty Records
 
 It is often useful to ignore sets of fields that the user has not filled in. You can control this by passing a `:reject_if` proc to `accepts_nested_attributes_for`. This proc will be called with each hash of attributes submitted by the form. If the proc returns `false` then Active Record will not build an associated object for that hash. The example below only tries to build an address if the `kind` attribute is set.