UPGRADING.INTERNALS: document ZEND_DECLARE_ATTRIBUTED_CONST (#19270)

[skip ci]

Author: DanielEScherzer

UPGRADING.INTERNALS

@@ -147,6 +147,11 @@ PHP 8.5 INTERNALS UPGRADE NOTES
 4. OpCode changes
 ========================
 
+* New ZEND_DECLARE_ATTRIBUTED_CONST is used when a global constant is declared
+  with `const` and has attributes; this opcode is used *instead* of the
+  ZEND_DECLARE_CONST, and in addition to the name of the constant and the
+  value to use, has a ZEND_OP_DATA with a pointer to the compiled attributes.
+
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 5. SAPI changes
 ========================