Add separate test to ensure that `delegate` with `:private` option returns correct value

Remove extra comments `# Asking for private method` in activesupport/test/core_ext/module_test.rb
Improve docs of using `delegate` with `:private`
Update changelog of #31944

activesupport/CHANGELOG.md

@@ -1,8 +1,7 @@
 ## Rails 6.0.0.alpha (Unreleased) ##
 
-*   Add `private: true` option to ActiveSupport's `delegate`.
-
-    In order to delegate methods as private methods:
+*   Add `:private` option to ActiveSupport's `Module#delegate`
+    in order to delegate methods as private:
 
         class User < ActiveRecord::Base
           has_one :profile
@@ -17,8 +16,6 @@
         # User.new.date_of_birth
         # => NoMethodError: private method `date_of_birth' called for #<User:0x00000008221340>
 
-    More information in #31944.
-
     *Tomas Valent*
 
 *   `String#truncate_bytes` to truncate a string to a maximum bytesize without

activesupport/lib/active_support/core_ext/module/delegation.rb

@@ -114,7 +114,7 @@ class Module
   #   invoice.customer_name    # => 'John Doe'
   #   invoice.customer_address # => 'Vimmersvej 13'
   #
-  # If you want the delegated method to be a private method,
+  # If you want the delegate methods to be a private,
   # use the <tt>:private</tt> option.
   #
   #   class User < ActiveRecord::Base
@@ -127,10 +127,10 @@ class Module
   #     end
   #   end
   #
-  #   User.new.age # 2
-  #   User.new.first_name # Tomas
-  #   User.new.date_of_birth # NoMethodError: private method `date_of_birth' called for #<User:0x00000008221340>
-  #   User.new.religion # NoMethodError: private method `religion' called for #<User:0x00000008221340>
+  #   User.new.age # => 2
+  #   User.new.first_name # => "Tomas"
+  #   User.new.date_of_birth # => NoMethodError: private method `date_of_birth' called for #<User:0x00000008221340>
+  #   User.new.religion # => NoMethodError: private method `religion' called for #<User:0x00000008221340>
   #
   # If the target is +nil+ and does not respond to the delegated method a
   # +Module::DelegationError+ is raised. If you wish to instead return +nil+,

activesupport/test/core_ext/module_test.rb

@@ -482,19 +482,29 @@ class ModuleTest < ActiveSupport::TestCase
       def initialize(place)
         @place = place
       end
+
+      delegate(:street, :city, to: :@place, prefix: :the, private: true)
     end
 
-    assert_equal %i(the_street the_city),
-      location.delegate(:street, :city, to: :@place, prefix: :the, private: true)
-
     place = location.new(Somewhere.new("Such street", "Sad city"))
 
-    assert_not_respond_to place, :street
-    assert_not_respond_to place, :city
-
     assert_not_respond_to place, :the_street
     assert place.respond_to?(:the_street, true)
     assert_not_respond_to place, :the_city
     assert place.respond_to?(:the_city, true)
   end
+
+  def test_delegate_with_private_option_returns_names_of_delegate_methods
+    location = Class.new do
+      def initialize(place)
+        @place = place
+      end
+    end
+
+    assert_equal [:street, :city],
+      location.delegate(:street, :city, to: :@place, private: true)
+
+    assert_equal [:the_street, :the_city],
+      location.delegate(:street, :city, to: :@place, prefix: :the, private: true)
+  end
 end