[docs] Add note about params hash

Newbies tends to misunderstand params hash is a plain old Hash. So I
added a note that params hash is ActionController::Parameters.

Author: JunichiIto

guides/source/action_controller_overview.md

@@ -110,7 +110,10 @@ class ClientsController < ApplicationController
 end
 ```
 
+NOTE: The `params` hash is not a plain old Ruby Hash; instead, it is an [`ActionController::Parameters`][] object. While it behaves like Hash, it does not inherit from Hash.
+
 [`params`]: https://api.rubyonrails.org/classes/ActionController/StrongParameters.html#method-i-params
+[`ActionController::Parameters`]: https://api.rubyonrails.org/classes/ActionController/Parameters.html
 
 ### Hash and Array Parameters