Fix https://issues.apache.org/bugzilla/show_bug.cgi?id=56383
Document ErrorReportValve's new property 'showReport' on the "security HowTO" page.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1593200 13f79535-47bb-0310-9956-ffa450edef68