update migration 'timestamps/1' docs to include ':null' option (#669)

Author: arcanemachine

lib/ecto/migration.ex

@@ -1283,8 +1283,8 @@ defmodule Ecto.Migration do
   @doc """
   Adds `:inserted_at` and `:updated_at` timestamp columns.
 
-  Those columns are of `:naive_datetime` type and by default cannot be null. A
-  list of `opts` can be given to customize the generated fields.
+  Those columns are of `:naive_datetime` type. A list of `opts` can be given
+  to customize the generated fields.
 
   Following options will override the repo configuration specified by
   `:migration_timestamps` option.
@@ -1300,6 +1300,8 @@ defmodule Ecto.Migration do
     * `:default` - the columns' default value. It can be a string, number, empty
       list, list of strings, list of numbers, or a fragment generated by
       `fragment/1`.
+    * `:null` - determines whether the column accepts null values. Defaults to
+      `false`.
 
   """
   def timestamps(opts \\ []) when is_list(opts) do