PHP 8.1 | MigrationGuide/New functions: add missing functions [1]

(mentioned in new features, but not in the new functions list)

> GD:
> * Avif support is now available through the `imagecreatefromavif()` and
>    `imageavif()` functions, if libgd has been built with avif support.

Refs:
* https://github.com/php/php-src/blob/f67986a9218f4889d9352a87c29337a5b6eaa4bd/UPGRADING#L245-L247
* php/php-src#7026
* php/php-src@81f6d36

Author: jrfnl

appendices/migration81/new-functions.xml

@@ -14,6 +14,23 @@
   </itemizedlist>
  </sect2>
 
+ <sect2 xml:id="migration81.new-functions.gd">
+  <title>GD</title>
+
+  <itemizedlist>
+   <listitem>
+    <simpara>
+     <function>imagecreatefromavif</function>
+    </simpara>
+   </listitem>
+   <listitem>
+    <simpara>
+     <function>imageavif</function>
+    </simpara>
+   </listitem>
+  </itemizedlist>
+ </sect2>
+
  <sect2 xml:id="migration81.new-functions.pcntl">
   <title>Process Control</title>