doc: add mysqlnd error code changes to UPGRADING

UPGRADING

@@ -126,6 +126,25 @@ PHP 8.4 UPGRADE NOTES
     has changed. Consult https://github.com/PCRE2Project/pcre2/blob/master/NEWS
     for a full changelog.
 
+- PDO_DBLIB:
+  . setAttribute, DBLIB_ATTR_STRINGIFY_UNIQUEIDENTIFIER and DBLIB_ATTR_DATETIME_CONVERT
+    have been changed to set value as a bool.
+
+- PDO_FIREBIRD:
+  . getAttribute, ATTR_AUTOCOMMIT has been changed to get the value as a bool.
+
+- PDO_MYSQL:
+  . getAttribute, ATTR_AUTOCOMMIT, ATTR_EMULATE_PREPARES, MYSQL_ATTR_DIRECT_QUERY have
+    been changed to get values as bool.
+
+- PDO_PGSQL:
+  . The DSN's credentials, when set, are given priority over their PDO
+    constructor counterparts, being closer to the documentation states.
+
+- MySQLnd
+  . The error code reported for MySQL server wait timeouts has been changed from 2006
+    to 4031 for MySQL server versions 8.0.24 and above.
+
 - PCNTL:
   . The functions pcntl_sigprocmask(), pcntl_sigwaitinfo() and
     pcntl_sigtimedwait() now throw: