docs: improve explanation

Apply suggestions from code review

Co-authored-by: Michal Sniatala <michal@sniatala.pl>

Author: kenjis

user_guide_src/source/changelogs/v4.3.4.rst

@@ -44,11 +44,11 @@ Forge::modifyColumn()
   to a bug, in previous versions, SQLite3/Postgres/SQLSRV might change
   ``NULL``/``NOT NULL`` unpredictably.
 - In previous versions, the OCI8 driver did not change ``NULL``/``NOT NULL``
-  when you don't specify ``null`` value.
+  when you don't specify the ``null`` key.
 - Now in all database drivers ``$forge->modifyColumn()`` always sets ``NULL``
-  when you don't specify ``null`` value.
+  when you don't specify the ``null`` key.
 - The ``NULL``/``NOT NULL`` change may still be unexpectedly, it is recommended
-  to always specifiy ``null`` value.
+  to always specify the ``null`` key.
 
 Message Changes
 ***************

user_guide_src/source/dbmgmt/forge.rst

@@ -297,7 +297,7 @@ 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. Unlike when creating
+    So it is recommended to always specify the value for ``null`` key. Unlike when creating
     a table, if ``null`` is not specified, the column will be ``NULL``, not
     ``NOT NULL``.
 

user_guide_src/source/installation/upgrade_434.rst

@@ -32,7 +32,7 @@ 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.
+specify the ``null`` key.
 
 Note that the bug may have changed unexpected NULL constraints in previous
 versions.