docs: add note for modifyColumn()

Author: kenjis

user_guide_src/source/dbmgmt/forge.rst

@@ -294,6 +294,15 @@ change the name, you can add a "name" key into the field defining array.
 
 .. literalinclude:: forge/026.php
 
+.. note:: The ``modifyColumn()`` may unexpectedly change ``NULL``/``NOT NULL``.
+    So it is recommended to always specify ``null`` value.
+
+.. note:: Due to a bug, prior v4.3.3, SQLite3 may not set ``NOT NULL`` even if you
+    specify ``'null' => false``.
+
+.. note:: Due to a bug, prior v4.3.3, Postgres and SQLSRV set ``NOT NULL`` even
+    if you specify ``'null' => false``.
+
 .. _db-forge-adding-keys-to-a-table:
 
 Adding Keys to a Table

user_guide_src/source/dbmgmt/forge/026.php

@@ -4,7 +4,8 @@
     'old_name' => [
         'name' => 'new_name',
         'type' => 'TEXT',
+        'null' => false,
     ],
 ];
 $forge->modifyColumn('table_name', $fields);
-// gives ALTER TABLE `table_name` CHANGE `old_name` `new_name` TEXT
+// gives ALTER TABLE `table_name` CHANGE `old_name` `new_name` TEXT NOT NULL