Merge pull request #7412 from kenjis/docs-add-upgrade_4xx

docs: improve Upgrading from 3.x to 4.x

Author: kenjis

user_guide_src/source/installation/upgrade_4xx.rst

@@ -62,7 +62,10 @@ Application Structure
 Routing
 =======
 
-- The Auto Routing is disabled by default. If you want to use the Auto Routing in the same way as CI3, you need to enable :ref:`auto-routing-legacy`.
+- The Auto Routing is disabled by default. You need to :ref:`define all routes
+  <defined-route-routing>` by default.
+- If you want to use the Auto Routing in the same way as CI3, you need to enable
+  :ref:`auto-routing-legacy`.
 - CI4 also has an optional new more secure :ref:`auto-routing-improved`.
 
 Model, View and Controller
@@ -204,6 +207,3 @@ Upgrading Libraries
     upgrade_sessions
     upgrade_validations
     upgrade_view_parser
-
-.. note::
-    More upgrade guides coming soon

user_guide_src/source/installation/upgrade_validations.rst

@@ -15,6 +15,9 @@ What has been changed
 =====================
 - 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.
+  Use :ref:`Rule Classes <validation-using-rule-classes>` or
+  :ref:`Closure Rule <validation-using-closure-rule>`
+  instead.
 - CI4 validation format rules do not permit empty string.
 - CI4 validation never changes your data.
 - Since v4.3.0, :php:func:`validation_errors()` has been introduced, but the API is different from CI3's.

user_guide_src/source/libraries/validation.rst

@@ -612,6 +612,8 @@ right after the name of the field the error should belong to::
 Creating Custom Rules
 *********************
 
+.. _validation-using-rule-classes:
+
 Using Rule Classes
 ==================