integrate the strong params README into the AC guide.

The current ActionController guide does not mention strong parameters
at all. I integrated the README into the guide to explain the API.

I also included a section to illustrate that the API does not solve
all possible whitelisting scenarios.

The origin was #9454.

guides/source/action_controller_overview.md

@@ -6,6 +6,7 @@ In this guide you will learn how controllers work and how they fit into the requ
 After reading this guide, you will know:
 
 * How to follow the flow of a request through a controller.
+* How to restrict parameters passed to your controller.
 * Why and how to store data in the session or cookies.
 * How to work with filters to execute code during request processing.
 * How to use Action Controller's built-in HTTP authentication.
@@ -170,6 +171,123 @@ These options will be used as a starting point when generating URLs, so it's pos
 
 If you define `default_url_options` in `ApplicationController`, as in the example above, it would be used for all URL generation. The method can also be defined in one specific controller, in which case it only affects URLs generated there.
 
+### Strong Parameters
+
+With strong parameters Action Controller parameters are forbidden to
+be used in Active Model mass assignments until they have been
+whitelisted. This means you'll have to make a conscious choice about
+which attributes to allow for mass updating and thus prevent
+accidentally exposing that which shouldn't be exposed.
+
+In addition, parameters can be marked as required and flow through a
+predefined raise/rescue flow to end up as a 400 Bad Request with no
+effort.
+
+```ruby
+class PeopleController < ActionController::Base
+  # This will raise an ActiveModel::ForbiddenAttributes exception
+  # because it's using mass assignment without an explicit permit
+  # step.
+  def create
+    Person.create(params[:person])
+  end
+
+  # This will pass with flying colors as long as there's a person key
+  # in the parameters, otherwise it'll raise a
+  # ActionController::MissingParameter exception, which will get
+  # caught by ActionController::Base and turned into that 400 Bad
+  # Request reply.
+  def update
+    person = current_account.people.find(params[:id])
+    person.update_attributes!(person_params)
+    redirect_to person
+  end
+
+  private
+    # Using a private method to encapsulate the permissible parameters
+    # is just a good pattern since you'll be able to reuse the same
+    # permit list between create and update. Also, you can specialize
+    # this method with per-user checking of permissible attributes.
+    def person_params
+      params.require(:person).permit(:name, :age)
+    end
+end
+```
+
+#### Permitted Scalar Values
+
+Given
+
+```ruby
+params.permit(:id)
+```
+
+the key `:id` will pass the whitelisting if it appears in `params` and
+it has a permitted scalar value associated. Otherwise the key is going
+to be filtered out, so arrays, hashes, or any other objects cannot be
+injected.
+
+The permitted scalar types are `String`, `Symbol`, `NilClass`,
+`Numeric`, `TrueClass`, `FalseClass`, `Date`, `Time`, `DateTime`,
+`StringIO`, `IO`, `ActionDispatch::Http::UploadedFile` and
+`Rack::Test::UploadedFile`.
+
+To declare that the value in `params+ must be an array of permitted
+scalar values map the key to an empty array:
+
+```ruby
+params.permit(:id => [])
+```
+
+To whitelist an entire hash of parameters, the `permit!+ method can be
+used
+
+```ruby
+params.require(:log_entry).permit!
+```
+
+This will mark the `:log_entry` parameters hash and any subhash of it
+permitted.  Extreme care should be taken when using `permit!` as it
+will allow all current and future model attributes to be
+mass-assigned.
+
+#### Nested Parameters
+
+You can also use permit on nested parameters, like:
+
+```ruby
+params.permit(:name, {:emails => []},
+              :friends => [ :name,
+                            { :family => [ :name ], :hobbies => [] }])
+```
+
+This declaration whitelists the `name`, `emails` and `friends`
+attributes. It is expected that `emails` will be an array of permitted
+scalar values and that `friends` will be an array of resources with
+specific attributes : they should have a `name` attribute (any
+permitted scalar values allowed), a `hobbies` attribute as an array of
+permitted scalar values, and a `family` attribute which is restricted
+to having a `name` (any permitted scalar values allowed, too).
+
+#### Outside the Scope of Strong Parameters
+
+The strong parameter API was designed with the most common use cases
+in mind. It is not meant as a silver bullet to handle all your
+whitelisting problems. However you can easily mix the API with your
+own code to adapt to your situation.
+
+Imagine a situation where you want to whitelist an attribute
+containing a hash with any keys. Using strong parameters you can't
+allow a hash with any keys but you can use a simple assignment to get
+the job done:
+
+```ruby
+def product_params
+  params.require(:product).permit(:name).tap do |whitelisted|
+    whitelisted[:data] = params[:product][:data]
+  end
+end
+```
 
 Session
 -------