Added note about zend_function.reserved[] fields usage

Author: dstogov

UPGRADING.INTERNALS

@@ -19,6 +19,7 @@ PHP 7.3 INTERNALS UPGRADE NOTES
   p. VM instruction operands (FETCH_CLASS, FETCH_CONSTANT, CATCH)
   q. sapi_cli_single_write()
   r. php_url
+  s. zend_function.reserved[]
 
 2. Build system changes
   a. Unix build system changes
@@ -129,6 +130,10 @@ PHP 7.3 INTERNALS UPGRADE NOTES
 
  r.  fields of php_url struct change from char * to zend_string *
 
+ s. Specual purose zend_functions marked by ZEND_ACC_CALL_VIA_TRAMPOLINE or
+    ZEND_ACC_FAKE_CLOSURE flags use reserved[0] for internal purpose.
+    Third party extensions must not modify reserved[] fileds of these functins.
+
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 2. Build system changes
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