Document IntlBreakIterator::getPartsIterator() $type change in UPGRADING

Follow-up to #21003. Documents the $type parameter change from string to int in UPGRADING.

Author: jordikroon

UPGRADING

@@ -37,6 +37,8 @@ PHP 8.6 UPGRADE NOTES
 - Intl:
   . Passing a non-stringable object as a time zone to Intl APIs that accept
     time zone objects or strings now raises a TypeError instead of an Error.
+  . The $type parameter of IntlBreakIterator::getPartsIterator() has been
+    changed from string to int to match the underlying implementation.
 
 - PCNTL:
   . pcntl_alarm() now raises a ValueError if the seconds argument is