Clarification on unit of measure of %D in mod_headers

PR: 47707
Reported by: Paschal Nee <pnee toombeola.com>


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@805763 13f79535-47bb-0310-9956-ffa450edef68