Clarify that exceptions cannot be cloned

Fixes #3418

Author: lacatoire

language/exceptions.xml

@@ -419,7 +419,7 @@ class Exception implements Throwable
 
     public function __construct($message = '', $code = 0, ?Throwable $previous = null);
 
-    final private function __clone();           // Inhibits cloning of exceptions.
+    private function __clone();                 // Inhibits cloning of exceptions.
 
     final public  function getMessage();        // message of exception
     final public  function getCode();           // code of exception

language/predefined/exception/clone.xml

@@ -12,10 +12,13 @@
    <modifier>private</modifier> <type>void</type><methodname>Exception::__clone</methodname>
    <void/>
   </methodsynopsis>
-  <para>
-   <classname>Exception</classname>s cannot be cloned,
-   and attempting to do so will throw an <classname>Error</classname>.
-  </para>
+  <warning>
+   <simpara>
+    <classname>Exception</classname>s cannot be cloned.
+    Attempting to clone an exception will throw an
+    <classname>Error</classname>.
+   </simpara>
+  </warning>
  </refsect1>
 
  <refsect1 role="parameters">
@@ -51,7 +54,10 @@
      <row>
       <entry>8.1.0</entry>
       <entry>
-       <methodname>Exception::__clone</methodname> is no longer final.
+       <methodname>Exception::__clone</methodname> is no longer
+       <modifier>final</modifier>. However, exceptions still cannot
+       be cloned; the <modifier>final</modifier> modifier was removed
+       because <literal>private final</literal> is logically redundant.
       </entry>
      </row>
     </tbody>