document HashWithIndifferentAccess#extractable_options?

activesupport/lib/active_support/hash_with_indifferent_access.rb

@@ -6,6 +6,8 @@ require 'active_support/core_ext/hash/keys'
 
 module ActiveSupport
   class HashWithIndifferentAccess < Hash
+    
+    # Always returns true, so that <tt>Array#extract_options!</tt> finds members of this class.
     def extractable_options?
       true
     end