Document Imagick::thumbnailImage params (fix #2539)

Author: vrana

reference/imagick/imagick/thumbnailimage.xml

@@ -56,6 +56,23 @@
       </para>
      </listitem>
     </varlistentry>
+    <varlistentry>
+     <term><parameter>fill</parameter></term>
+     <listitem>
+      <para>
+       If the image does not fill the box completely then the box is filled with
+       image's background color.
+      </para>
+     </listitem>
+    </varlistentry>
+    <varlistentry>
+     <term><parameter>legacy</parameter></term>
+     <listitem>
+      <para>
+       Round the smaller dimension down instead to the closest integer.
+      </para>
+     </listitem>
+    </varlistentry>
    </variablelist>
   </para>
  </refsect1>