flags.xml: Add examples and security warnings for UnsafeAllow3F and UnsafePrefixStat
Both flags were introduced to address CVE-2024-38474 and CVE-2024-38475
respectively. Add practical examples showing common scenarios where each
flag is needed (PHP front controller for UnsafeAllow3F; backreference-
starting substitution for UnsafePrefixStat), along with warning notes
linking to the relevant CVEs and advising restraint in their use.
git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@
1934248 13f79535-47bb-0310-9956-
ffa450edef68