Add missing documentation for secret.

      <tr><td>?req_attribute</td><td>0x0A</td><td>String</td><td>Name (the name of the

      attribute follows)</td></tr>

      <tr><td>?ssl_key_size</td><td>0x0B</td><td>Integer</td><td></td></tr>

      <tr><td>?secret</td><td>0x0C</td><td>String</td><td>Sent if secret is configured</td></tr>

      <tr><td>are_done</td><td>0xFF</td><td>-</td><td>request_terminator</td></tr>

    </table>

    <p>The <code>context</code> and <code>servlet_path</code> are not

    username and the type of authentication used to establish their identity

    (e.g. Basic, Digest).</p>

    <p>The <code>query_string</code>, <code>ssl_cert</code>,

    <code>ssl_cipher</code>, and <code>ssl_session</code> refer to the

    <code>ssl_cipher</code>, <code>ssl_session</code> and

    <code>ssl_key_size</code> refer to the

    corresponding pieces of HTTP and HTTPS.</p>

    <p>The <code>jvm_route</code>, is used to support sticky

    sessions -- associating a user's sesson with a particular Tomcat instance

    A pair of strings to represent the attribute name and value are sent

    immediately after each instance of that code.  Environment values are passed

    in via this method.</p>

    <p>The <code>secret</code> is sent when the <code>secret=secret_keyword</code> parameter is

    used in

    <directive module="mod_proxy">ProxyPass</directive> or

    <directive module="mod_proxy">BalancerMember</directive> directives.

    The backend needs to support secret and the values must match.

    <code>request.secret</code> or <code>requiredSecret</code> are documented in the AJP

    configuration of the Apache Tomcat.</p>

    <p>Finally, after all the attributes have been sent, the attribute

    terminator, <code>0xFF</code>, is sent.  This signals both the end of the

    list of attributes and also then end of the Request Packet.</p>