Add note to modify/3 docs about avoiding type mods (#353)

Author: aegatlin

lib/ecto/migration.ex

@@ -1028,6 +1028,12 @@ defmodule Ecto.Migration do
 
   See `add/3` for more information on supported types.
 
+  If you want to modify a column without changing its type,
+  such as adding or dropping a null constraints, consider using
+  the `execute/2` command with the relevant SQL command instead
+  of `modify/3`, if supported by your database. This may avoid
+  redundant type updates and be more efficient.
+
   ## Examples
 
       alter table("posts") do