Add/show the options item ":default" on the @doc block. (#404)

Co-authored-by: Julian Somoza <julian@animus.com.ar>

Author: somoza

lib/ecto/migration.ex

@@ -1026,6 +1026,9 @@ defmodule Ecto.Migration do
       Setting it to `false` disables the column.
     * `:type` - the type of the `:inserted_at` and `:updated_at` columns.
       Defaults to `:naive_datetime`.
+    * `: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`.
 
   """
   def timestamps(opts \\ []) when is_list(opts) do