imagewebp: Document what quality -1 means (fix #3118)

Author: vrana

reference/image/functions/imagewebp.xml

@@ -36,6 +36,7 @@
       <para>
        <parameter>quality</parameter> ranges from 0 (worst
        quality, smaller file) to 100 (best quality, biggest file).
+       If <literal>-1</literal> is provided, the default value <literal>80</literal> is used.
       </para>
      </listitem>
     </varlistentry>