Update list of `duplicable?` objects in AS core_ext guide [ci skip]

Complex and Rational objects can not be duplicated.

Duplicable objects in Ruby 2.4.0 include:
- most numbers
- symbols
- `nil`
- `true`
- `false`

1 files changed, 31 insertions, 17 deletions

diff --git a/guides/source/active_support_core_extensions.md b/guides/source/active_support_core_extensions.md
index 6bbc79a326..bfe6b7b7ea 100644
--- a/guides/source/active_support_core_extensions.md
+++ b/guides/source/active_support_core_extensions.md
@@ -135,36 +135,50 @@ NOTE: Defined in `active_support/core_ext/object/blank.rb`.
 
 ### `duplicable?`
 
-A few fundamental objects in Ruby are singletons. For example, in the whole life of a program the integer 1 refers always to the same instance:
+In Ruby 2.4 most objects can be duplicated throuh `dup` or `clone`, except 
+method objects and certain numbers. Ruby 2.2 and 2.3 can't duplicate `nil`,
+`false`, `true`, and  symbols, as well as instances of `Float`, `Fixnum` 
+and `Bignum`.
 
 ```ruby
-1.object_id                 # => 3
-Math.cos(0).to_i.object_id  # => 3
+"foo".dup           # => "foo"
+"".dup              # => ""
+1.method(:+).dup    # => TypeError: allocator undefined for Method
+Complex(0).dup      # => TypeError: can't copy Complex
 ```
 
-Hence, there's no way these objects can be duplicated through `dup` or `clone`:
+Active Support provides `duplicable?` to programmatically query an object about 
+this property:
 
 ```ruby
-true.dup  # => TypeError: can't dup TrueClass
+"foo".duplicable?           # => true
+"".duplicable?              # => true
+Rational(1).duplicable?     # => false
+Complex(1).duplicable?      # => false
+1.method(:+).duplicable?    # => false
 ```
 
-Some numbers which are not singletons are not duplicable either:
+`duplicable?` returns the correct result for the version of Ruby used.
 
 ```ruby
-0.0.clone        # => allocator undefined for Float
-(2**1024).clone  # => allocator undefined for Bignum
-```
+# version 2.4
+nil.dup                 # => nil
+:my_symbol.dup          # => :my_symbol
+1.dup                   # => 1
 
-Active Support provides `duplicable?` to programmatically query an object about this property:
+nil.duplicable?         # => true
+:my_symbol.duplicable?  # => true
+1.duplicable?           # => true
 
-```ruby
-"foo".duplicable? # => true
-"".duplicable?    # => true
-0.0.duplicable?   # => false
-false.duplicable? # => false
-```
+# version 2.2 and 2.3
+nil.dup                 # => TypeError: can't dup NilClass
+:my_symbol.dup          # => TypeError: can't dup Symbol
+1.dup                   # => TypeError: can't dup Fixnum
 
-By definition all objects are `duplicable?` except `nil`, `false`, `true`, symbols, numbers, class, module, and method objects.
+nil.duplicable?         # => false
+:my_symbol.duplicable?  # => false
+1.duplicable?           # => false
+```
 
 WARNING: Any class can disallow duplication by removing `dup` and `clone` or raising exceptions from them. Thus only `rescue` can tell whether a given arbitrary object is duplicable. `duplicable?` depends on the hard-coded list above, but it is much faster than `rescue`. Use it only if you know the hard-coded list is enough in your use case.