a section for mod_authz_svn directives to the reference chapter.
* en/book/ch09-reference.xml
(svn.ref.mod_dav_svn): Fix the SVNPathAuthz directive to not claim
to have a file-path value -- it's values are "On", "Off", or (the
newly-added) "short_circuit".
(svn.ref.mod_authz_svn): New section for mod_authz_svn directives.
