PHP 8.1 | MigrationGuide/Deprecated: add missing function

> GD:
> The `$num_points` parameter of `image(open|filled)polygon` has been deprecated.

Refs:
* https://github.com/php/php-src/blob/f67986a9218f4889d9352a87c29337a5b6eaa4bd/UPGRADING#L397-L398
* php/php-src#6789
* php/php-src@e1285c4

Author: mumumu

appendices/migration81/deprecated.xml

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="utf-8"?>
 <!-- $Revision$ -->
-<!-- EN-Revision: 9b7ab9bc8e78b1201b8b8c71eac051256d433993 Maintainer: mumumu Status: ready -->
+<!-- EN-Revision: 7d6c8735cc1adb853081ceb854931dc1cc41dfb7 Maintainer: mumumu Status: ready -->
 <sect1 xml:id="migration81.deprecated">
  <title>PHP 8.1.x で推奨されなくなる機能</title>
 
@@ -236,7 +236,8 @@ $arr[] = 2;
   <title>GD</title>
 
   <para>
-   <function>imageopenpolygon</function> と
+   <function>imagepolygon</function>,
+   <function>imageopenpolygon</function>,
    <function>imagefilledpolygon</function>
    の
    <parameter>num_points</parameter> 引数は、