language: document the PHP 8.4 comparison and readonly clone BC breaks

[lacatoire]

Documents the two remaining PHP 8.4 core language BC breaks:

• Indirect modification of a readonly property within __clone() is no longer allowed, for example acquiring a reference to it. Added as a note to the readonly properties section, right after the PHP 8.3 clone reinitialization paragraph.
• Encountering recursion during comparison now throws an Error instead of raising a fatal error. Added as a note to the comparison operators page.

Tracker: both items are on the PHP 8.4 Documentation Tracker, listed as "(BC Break) Indirect Modification of readonly Properties" and "(BC Break) Recursion during comparison".

Sources:

• Indirect modification of readonly properties within __clone(): UPGRADING for PHP 8.4, php-src #15012 (commit 46ee0fb), test Zend/tests/readonly_props/readonly_clone_error7.phpt.
• Recursion during comparison: UPGRADING for PHP 8.4, php-src #14989 (commit 82479e8), implementation in Zend/zend_hash.c and Zend/zend_object_handlers.c, test Zend/tests/recursive_array_comparison.phpt.

Verified on php:8.3-cli and php:8.4-cli:

class A {
    public readonly array $p;
    public function __construct(array $p) { $this->p = $p; }
    public function __clone(): void { $r = &$this->p; $r[] = 1; }
}
clone new A([0]);
// 8.3: succeeds, the property is modified
// 8.4: Error: Cannot indirectly modify readonly property A::$p

$a = [&$a];
var_dump($a === [[]]);
// 8.3: Fatal error: Nesting level too deep - recursive dependency?
// 8.4: Error: Nesting level too deep - recursive dependency? (catchable)

The comparison note deliberately says "encountering recursion during comparison" rather than referring to a maximum nesting level: the check is a recursion guard, and a single self-referencing value compared against a plain one is enough to trigger it. Comparing a value with itself ($a === $a) still returns true.

[kamil-tekiela]

LGTM