Fix mod_asis docs re: use of HTTP/CGI headers.

    <p>The contents of any file with a <code>.asis</code> extension

    will then be sent by Apache to the client with almost no

    changes. Clients will need HTTP headers to be attached, so do

    not forget them. A Status: header is also required; the data

    should be the 3-digit HTTP response code, followed by a textual

    message.</p>

    changes. In particular, HTTP headers are derived from the file

    itself according to <module>mod_cgi</module> rules, so an asis

    file must include valid headers, and may also use the CGI

    Status: header to determine the HTTP response code.</p>

    <p>Here's an example of a file whose contents are sent <em>as

    is</em> so as to tell the client that a file has