Update Migration.rename/3 docs. (#356)

Author: aegatlin

lib/ecto/migration.ex

@@ -965,7 +965,9 @@ defmodule Ecto.Migration do
   end
 
   @doc """
-  Renames a column outside of the `alter` statement.
+  Renames a column.
+
+  Note that this occurs outside of the `alter` statement.
 
   ## Examples