Updated comment about features of fixtures

 - Added a note about some features which are not dependent on
   auto-generated ID's.
 - [ci skip]

activerecord/lib/active_record/fixtures.rb

@@ -181,6 +181,9 @@ class FixtureClassNotFound < ActiveRecord::ActiveRecordError #:nodoc:
   # * Stable, autogenerated IDs
   # * Label references for associations (belongs_to, has_one, has_many)
   # * HABTM associations as inline lists
+  #
+  # There are some more advanced features available even if the id is specified:
+  #
   # * Autofilled timestamp columns
   # * Fixture label interpolation
   # * Support for YAML defaults