Clarify "GD image" in imagegd() synopsis

The input is a GdImage (i.e. an object), and the output is a GD image (i.e. data in GD format)

Author: IMSoP

reference/image/functions/imagegd.xml

@@ -13,7 +13,7 @@
    <methodparam choice="opt"><type class="union"><type>string</type><type>null</type></type><parameter>file</parameter><initializer>&null;</initializer></methodparam>
   </methodsynopsis>
   <para>
-   Outputs a GD image to the given <parameter>file</parameter>.
+   Outputs or saves the given <parameter>image</parameter in GD format.
   </para>
  </refsect1>
  <refsect1 role="parameters">