Improve PDO entry in UPGRADING file (#18446)

Author: kamil-tekiela

UPGRADING

@@ -82,8 +82,10 @@ PHP 8.5 UPGRADE NOTES
     emitted.
     To pass a variable by-ref to a constructor argument use the general
     array value reference assignment: $ctor_args = [&$valByRef]
-  . Attempting to modify a PDOStatement during a call to PDO::fetch(),
-    PDO::fetchObject(), PDO::fetchAll() will now throw an Error.
+  . Attempting to call PDOStatement::setFetchMode during a call to PDO::fetch(),
+    PDO::fetchObject(), PDO::fetchAll(),
+    for example using tricks such as passing the statement object as a constructor
+    argument when fetching into an object, will now throw an Error.
   . The value of the constants PDO::FETCH_GROUP, PDO::FETCH_UNIQUE,
     PDO::FETCH_CLASSTYPE, PDO::FETCH_PROPS_LATE, and PDO::FETCH_SERIALIZE
     has changed.