ext/intl: correct the 8.4.0 Error normalization, it only concerns cloning (#5798)

Only the clone path changed in 8.4.0. Throwing an Error when a method is
called on a non-initialized Intl object is not new: the macros doing so
date back to 2012 and PHP 8.3 behaves identically. Saying the classes
"always throw" is also falsifiable on 8.4: UConverter::convert() returns
false, ResourceBundle::get() returns null and Collator::getAttribute()
returns -1 on a non-initialized object.

What 8.4.0 changed is the clone handlers, and both of their branches
matter: refusing to clone a non-initialized object, and an initialized
object whose ICU clone() call fails. Formerly most classes threw a base
Exception there, and Spoofchecker raised an uncatchable fatal error.

- reference/intl/book.xml: restate the note accordingly
- appendices/migration84/other-changes.xml: fix the same overstatement in
  the migration entry so both places agree

Sources
- php-src 33f1cf202ef ("ext/intl: Various refactoring", php-src#14360):
  "Normalize cloning error handling behaviour - Always throw a Error
  exception as we cannot progress from here". Only clone paths and idn.c
  are touched, no INTL_METHOD_CHECK_STATUS macro.
- UPGRADING (PHP 8.4) section 9, added by cd44826c1af
- Pre-existing Error on use: a5d0c1e21b (2012-08-21), f5b421621d (2012-05-31)

Author: lacatoire

appendices/migration84/other-changes.xml

@@ -551,9 +551,12 @@
    <title>Intl</title>
 
    <simpara>
-    The behaviour of Intl class has been normalized to always throw
-    <exceptionname>Error</exceptionname> exceptions when attempting to use
-    a non-initialized object, or when cloning fails.
+    Cloning an Intl object consistently throws an
+    <exceptionname>Error</exceptionname> when the clone cannot be performed,
+    either because the object has not been initialized, or because the
+    underlying ICU clone operation failed. Formerly, most of these classes
+    threw an <exceptionname>Exception</exceptionname> instead, and
+    <classname>Spoofchecker</classname> raised a fatal error.
    </simpara>
   </sect3>
 

reference/intl/book.xml

@@ -93,6 +93,18 @@
    </listitem>
   </itemizedlist>
 
+  <note>
+   <simpara>
+    As of PHP 8.4.0, cloning an Intl object consistently throws an
+    <exceptionname>Error</exceptionname> when the clone cannot be performed,
+    either because the object has not been initialized (created without
+    invoking its constructor), or because the underlying ICU clone operation
+    failed. Formerly, most of these classes threw an
+    <exceptionname>Exception</exceptionname> instead, and
+    <classname>Spoofchecker</classname> raised a fatal error.
+   </simpara>
+  </note>
+
   <!-- {{{ Links -->
   <section xml:id="intl.links">
    <title>Links</title>