Add few lines to describe how aggregation part caching works. [ci skip]

activerecord/lib/active_record/aggregations.rb

@@ -86,6 +86,10 @@ def clear_aggregation_cache #:nodoc:
     #   customer.address_street = "Hyancintvej"
     #   customer.address_city   = "Copenhagen"
     #   customer.address        # => Address.new("Hyancintvej", "Copenhagen")
+    #   customer.address_street = "Vesterbrogade"
+    #   customer.address        # => Address.new("Hyancintvej", "Copenhagen")
+    #   customer.clear_aggregation_cache
+    #   customer.address        # => Address.new("Vesterbrogade", "Copenhagen")
     #   customer.address = Address.new("May Street", "Chicago")
     #   customer.address_street # => "May Street"
     #   customer.address_city   # => "Chicago"