docs: add upgrade note

Author: kenjis

user_guide_src/source/changelogs/v4.3.4.rst

@@ -35,6 +35,8 @@ Redirect Status Code
   always be used when you don't specify a status code. In previous versions,
   302 might be changed.
 
+.. _v434-forge-modifycolumn:
+
 Forge::modifyColumn()
 ---------------------
 

user_guide_src/source/installation/upgrade_434.rst

@@ -25,6 +25,18 @@ Redirect Status Code
   :ref:`ChangeLog v4.3.4 <v434-redirect-status-code>` and if the code is not
   what you want, :ref:`specify status codes <response-redirect-status-code>`.
 
+Forge::modifyColumn() and NULL
+==============================
+
+A bug fix may have changed the NULL constraint in the result of
+:ref:`$forge->modifyColumn() <db-forge-modifyColumn>`. See
+:ref:`Change Log <v434-forge-modifycolumn>`.
+To set the desired NULL constraint, change ``Forge::modifyColumn()`` to always
+specify the ``null`` value.
+
+Note that the bug may have changed unexpected NULL constraints in previous
+versions.
+
 Breaking Enhancements
 *********************