Add documentation for DNS issues (reliability and security), and try

to explain the virtual host matching process.

Try to continue gracefully by disabling the vhost if a DNS lookup
fails while parsing the configuration file.

Reviewed by:	Roy, Jim
Submitted by:
Obtained from:


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@77744 13f79535-47bb-0310-9956-ffa450edef68