Revert "dont encourage AC::Parameters#permit_all_parameters usage [ci skip]"

This reverts commit cd17ee55.

Reason: Let's note that this is discouraged, not remove the docs

[ci skip]

actionpack/lib/action_controller/metal/strong_parameters.rb

@@ -43,6 +43,19 @@ def initialize(param) # :nodoc:
   #   Person.first.update_attributes!(permitted)
   #   # => #<Person id: 1, name: "Francesco", age: 22, role: "user">
   #
+  # It provides a +permit_all_parameters+ option that controls the top-level
+  # behaviour of new instances. If it's +true+, all the parameters will be
+  # permitted by default. The default value for +permit_all_parameters+
+  # option is +false+.
+  #
+  #   params = ActionController::Parameters.new
+  #   params.permitted? # => false
+  #
+  #   ActionController::Parameters.permit_all_parameters = true
+  #
+  #   params = ActionController::Parameters.new
+  #   params.permitted? # => true
+  #
   # <tt>ActionController::Parameters</tt> is inherited from
   # <tt>ActiveSupport::HashWithIndifferentAccess</tt>, this means
   # that you can fetch values using either <tt>:key</tt> or <tt>"key"</tt>.
@@ -55,7 +68,8 @@ class Parameters < ActiveSupport::HashWithIndifferentAccess
     attr_accessor :permitted # :nodoc:
 
     # Returns a new instance of <tt>ActionController::Parameters</tt>.
-    # Also, sets the +permitted+ attribute to +false+.
+    # Also, sets the +permitted+ attribute to the default value of
+    # <tt>ActionController::Parameters.permit_all_parameters</tt>.
     #
     #   class Person
     #     include ActiveRecord::Base
@@ -64,6 +78,12 @@ class Parameters < ActiveSupport::HashWithIndifferentAccess
     #   params = ActionController::Parameters.new(name: 'Francesco')
     #   params.permitted?  # => false
     #   Person.new(params) # => ActiveModel::ForbiddenAttributesError
+    #
+    #   ActionController::Parameters.permit_all_parameters = true
+    #
+    #   params = ActionController::Parameters.new(name: 'Francesco')
+    #   params.permitted?  # => true
+    #   Person.new(params) # => #<Person id: nil, name: "Francesco">
     def initialize(attributes = nil)
       super(attributes)
       @permitted = self.class.permit_all_parameters