Fix #81423: UPGRADING doesn't mention that readonly becomes a reserved keyword

appendices/migration81/incompatible.xml

@@ -69,6 +69,13 @@ var_dump(B::counter()); // int(4), previously int(2)
    </para>
   </sect3>
 
+  <sect3 xml:id="migration81.incompatible.core.new-keywords">
+   <title>New Keywords</title>
+   <para>
+    <literal>readonly</literal> is a keyword now. However, it still may be used
+    as function name.
+   </para>
+  </sect3>
  </sect2>
 
  <sect2 xml:id="migration81.incompatible.resource2object">

appendices/reserved.xml

@@ -215,6 +215,9 @@
        </entry>
       </row>
       <row>
+       <entry>
+        <link linkend="language.oop5.properties.readonly-properties">readonly</link> (as of PHP 8.1.0) *
+       </entry>
        <entry>
         <function>require</function>
        </entry>
@@ -227,11 +230,11 @@
        <entry>
         <link linkend="language.variables.scope">static</link>
        </entry>
+      </row>
+      <row>
        <entry>
         <link linkend="control-structures.switch">switch</link>
        </entry>
-      </row>
-      <row>
        <entry>
         <link linkend="language.exceptions">throw</link>
        </entry>
@@ -244,11 +247,11 @@
        <entry>
         <function>unset</function>
        </entry>
+      </row>
+      <row>
        <entry>
         <link linkend="language.namespaces">use</link>
        </entry>
-      </row>
-      <row>
        <entry>
         <link linkend="language.oop5.properties">var</link>
        </entry>
@@ -261,14 +264,24 @@
        <entry>
         <link linkend="language.generators">yield</link>
        </entry>
+      </row>
+      <row>
        <entry>
         <link linkend="control-structures.yield.from">yield from</link>
        </entry>
+       <entry></entry>
+       <entry></entry>
+       <entry></entry>
+       <entry></entry>
       </row>
      </tbody>
     </tgroup>
    </table>
-   
+
+   <simpara>
+    * <literal>readonly</literal> may be used as function name.
+   </simpara>
+
    <table>
     <title>Compile-time constants</title>
     <tgroup cols="5">