docs: add links to other pages

Author: kenjis

user_guide_src/source/installation/upgrade_database.rst

@@ -14,7 +14,8 @@ Documentations
 What has been changed
 =====================
 - The functionality in CI3 is basically the same as in CI4.
-- The method names have changed to camelCase and the query builder now needs to be initialized before you can run queries on it.
+- The method names have changed to camelCase and the :doc:`Query Builder <../database/query_builder>`
+  now needs to be initialized before you can run queries on it.
 
 Upgrade Guide
 =============

user_guide_src/source/installation/upgrade_security.rst

@@ -12,7 +12,7 @@ Documentations
 - :doc:`Security Documentation CodeIgniter 4.X </libraries/security>`
 
 .. note::
-    If you use the :doc:`form helper </helpers/form_helper>` and enable the CSRF filter globally, then ``form_open()`` will automatically insert a hidden CSRF field in your forms. So you do not have to upgrade this by yourself.
+    If you use the :doc:`../helpers/form_helper` and enable the CSRF filter globally, then :php:func:`form_open()` will automatically insert a hidden CSRF field in your forms. So you do not have to upgrade this by yourself.
 
 What has been changed
 =====================

user_guide_src/source/installation/upgrade_validations.rst

@@ -13,7 +13,7 @@ Documentations of Library
 
 What has been changed
 =====================
-- If you want to change validation error display, you have to set CI4 validation View templates.
+- If you want to change validation error display, you have to set CI4 :ref:`validation View templates <validation-customizing-error-display>`.
 - CI4 validation has no Callbacks nor Callable in CI3.
 - CI4 validation format rules do not permit empty string.
 - CI4 validation never changes your data.

user_guide_src/source/libraries/validation.rst

@@ -516,6 +516,8 @@ When specifying a field with a wildcard, all errors matching the mask will be ch
 
 .. literalinclude:: validation/029.php
 
+.. _validation-customizing-error-display:
+
 Customizing Error Display
 *************************