Add UPGRADING.INTERNALS entry for ZEND_NONSTRING

[ci skip]

Author: nielsdos

UPGRADING.INTERNALS

@@ -31,6 +31,9 @@ PHP 8.5 INTERNALS UPGRADE NOTES
     runtime.
   . Removed the cache_slot argument of zend_check_user_type_slow() because
     now it only relies on the CE cache.
+  . Added ZEND_NONSTRING attribute macro for character arrays that do not
+    represent strings. This allows to silence the GCC 15.x
+    `-Wunterminated-string-initialization` warning.
 
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