adding examples to deep_dup method

activesupport/lib/active_support/core_ext/object/deep_dup.rb

 class Object
-  # Returns a deep copy of object if it's duplicable.
+  # Returns a deep copy of object if it's duplicable. If it's
+  # not duplicable, returns +self+.
+  #
+  #   object = Object.new
+  #   dup    = object.deep_dup
+  #   dup.instance_variable_set(:@a, 1)
+  #
+  #   object.instance_variable_defined?(:@a) #=> false
+  #   dup.instance_variable_defined?(:@a)    #=> true
   def deep_dup
     duplicable? ? dup : self
   end
@@ -7,6 +15,13 @@ def deep_dup
 
 class Array
   # Returns a deep copy of array.
+  #
+  #   array = [1, [2, 3]]
+  #   dup   = array.deep_dup
+  #   dup[1][2] = 4
+  #
+  #   array[1][2] #=> nil
+  #   dup[1][2]   #=> 4
   def deep_dup
     map { |it| it.deep_dup }
   end
@@ -14,6 +29,13 @@ def deep_dup
 
 class Hash
   # Returns a deep copy of hash.
+  #
+  #   hash = { a: { b: 'b' } }
+  #   dup  = hash.deep_dup
+  #   dup[:a][:c] = 'c'
+  #
+  #   hash[:a][:c] #=> nil
+  #   dup[:a][:c]  #=> "c"
   def deep_dup
     each_with_object(dup) do |(key, value), hash|
       hash[key.deep_dup] = value.deep_dup