Some doc updates realted to ssi.

1. Add info on how to activate ssi to upgrading.html.

2. Remove mention of the ssi magic mime-types, since they no
longer work.

3. Place a warning on xbithack that it doesn't currently work,
and reference the bug report.

4. Add a warning about mod_include parsing non-html content.

5. Misc little fixups.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@89655 13f79535-47bb-0310-9956-ffa450edef68