Fix https://issues.apache.org/bugzilla/show_bug.cgi?id=56383
Merged r1590451 from tomcat/trunk:
Document the new feature on the "security HowTO" page.
Move error valve documentation lower in the valves list
(access log valves and access control valves should be the first ones there).

git-svn-id: https://svn.apache.org/repos/asf/tomcat/tc7.0.x/trunk@1590453 13f79535-47bb-0310-9956-ffa450edef68