new-features.xml: make the example live and add the heredoc variant (#3585)

appendices/migration70/new-features.xml

@@ -269,9 +269,18 @@ object(class@anonymous)#2 (0) {
   <informalexample>
    <programlisting role="php">
 <![CDATA[
-echo "\u{aa}";
-echo "\u{0000aa}";
-echo "\u{9999}";
+<?php
+
+echo "\u{aa}", PHP_EOL;
+echo "\u{0000aa}", PHP_EOL;
+
+echo "\u{9999}", PHP_EOL;
+
+echo <<<EOT
+\u{01f418}
+EOT;
+
+?>
 ]]>
    </programlisting>
    &example.outputs;