Fix https://issues.apache.org/bugzilla/show_bug.cgi?id=55746
Add documentation for AllRolesMode and CombinedRealm
Patch by Cédric Couralet

git-svn-id: https://svn.apache.org/repos/asf/tomcat/tc6.0.x/trunk@1540376 13f79535-47bb-0310-9956-ffa450edef68