add "full-upgrade" to the apt.5.xml documentation

doc/apt.8.xml

@@ -110,6 +110,13 @@
      </para></listitem>
      </varlistentry>
 
+     <varlistentry><term><option>full-upgrade</option></term>
+     <listitem><para><literal>full-upgrade</literal> performs the
+     function of upgrade but may also remove installed packages
+     if that is required in order to resolve a package conflict.
+     </para></listitem>
+     </varlistentry>
+
 
    </variablelist>
  </refsect1>