Fix #79809: imagecreatefromwebp() does not support animated WebP

reference/image/functions/imagecreatefromwebp.xml

@@ -15,6 +15,7 @@
   <para>
    <function>imagecreatefromwebp</function> returns an image identifier
    representing the image obtained from the given filename.
+   Note that animated WebP files cannot be read.
   </para>
   &tip.fopen-wrapper;
  </refsect1>