docs: fix RST format and improve the description

Author: kenjis

user_guide_src/source/installation/upgrade_4xx.rst

@@ -171,13 +171,21 @@ Error Handling
 - The behavior in CI4 has been slightly changed.
 
   - In CI3 the behavior was set in the **index.php** file:
-      - errors ignored by ``error_reporting()`` was not logged;
-      - other errors were logged (depending on the settings in ``log_threshold``, they may not have been written to the log file);
-      - errors with an error level of ``E_ERROR | E_PARSE | E_COMPILE_ERROR | E_CORE_ERROR | E_USER_ERROR`` stopped framework processing, regardless of the error level set in ``error_reporting()``;
+
+      - errors with the error level set by ``error_reporting()`` are logged (but
+        depending on the ``log_threshold`` setting, they may not be written to
+        the log file).
+      - errors with an error level of
+        ``E_ERROR | E_PARSE | E_COMPILE_ERROR | E_CORE_ERROR | E_USER_ERROR``
+        stopped framework processing, regardless of the error level set in
+        ``error_reporting()``.
   - In CI4, the behavior is set in the **app/Config/Boot/{environment}.php** file:
-      - errors with the error level set by ``error_reporting()`` are logged (but depending on the
-    ``Config\Logger::$threshold`` setting, they may not be written to the log file);
-      - all errors that are not ignored by ``error_reporting()`` will stop the framework processing;
+
+      - errors with the error level set by ``error_reporting()`` are logged (but
+        depending on the ``Config\Logger::$threshold`` setting, they may not be
+        written to the log file).
+      - all errors that are not ignored by ``error_reporting()`` will stop the
+        framework processing.
 
 Extending the Framework
 =======================