Wording + Markup and add extending constraint

Author: Girgias

language/oop5/basic.xml

@@ -130,10 +130,10 @@ Stack trace:
     <title>Readonly classes</title>
     <para>
      As of PHP 8.2.0, a class can be marked with the
-     <literal>readonly</literal> modifier.
-     Marking a class as <literal>readonly</literal> will add the
-     <link linkend="language.oop5.properties.readonly-properties"><literal>readonly</literal> modifier to every declared property</link>,
-     and prevent the creation of
+     <modifier>readonly</modifier> modifier.
+     Marking a class as <modifier>readonly</modifier> will add the
+     <link linkend="language.oop5.properties.readonly-properties"><modifier>readonly</modifier> modifier</link>
+     to every declared property, and prevent the creation of
      <link linkend="language.oop5.properties.dynamic-properties">dynamic properties</link>.
      Moreover, it is impossible to add support for them by using the
      <classname>AllowDynamicProperties</classname> attribute. Attempting to do so
@@ -157,8 +157,9 @@ readonly class Foo {
      As neither untyped, nor static properties can be marked with the
      <literal>readonly</literal> modifier, readonly classes cannot declare
      them either:
-     <example>
-      <programlisting role="php">
+    </para>
+    <example>
+     <programlisting role="php">
 <![CDATA[
 <?php
 readonly class Foo
@@ -169,8 +170,8 @@ readonly class Foo
 // Fatal error: Readonly property Foo::$bar must have type
 ?>
 ]]>
-      </programlisting>
-      <programlisting role="php">
+     </programlisting>
+     <programlisting role="php">
 <![CDATA[
 <?php
 readonly class Foo
@@ -181,8 +182,13 @@ readonly class Foo
 // Fatal error: Readonly class Foo cannot declare static properties
 ?>
 ]]>
-      </programlisting>
-     </example>
+     </programlisting>
+    </example>
+    <para>
+     A <modifier>readonly</modifier> class can be
+     <link linkend="language.oop5.basic.extends">extended</link>
+     if, and only if, the child class is also a
+     <modifier>readonly</modifier> class.
     </para>
    </sect3>
   </sect2>