docs: EUC_TW can be up to four bytes wide, not three

Backpatch-through: 13
Security: CVE-2025-1094

Author: anarazel

doc/src/sgml/charset.sgml

@@ -1871,7 +1871,7 @@ ORDER BY c COLLATE ebcdic;
          <entry>Traditional Chinese, Taiwanese</entry>
          <entry>Yes</entry>
          <entry>Yes</entry>
-         <entry>1&ndash;3</entry>
+         <entry>1&ndash;4</entry>
          <entry></entry>
         </row>
         <row>