backport:

add some explanations how to use AOFBT correctly.

related to: PR 15045
Reviewed by: Astrid Ke�ler <kess@kess-net.de>


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/APACHE_2_0_BRANCH@97796 13f79535-47bb-0310-9956-ffa450edef68