Add document for sessionIdAttribute attribute in JvmRouteBinderValve.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1527468 13f79535-47bb-0310-9956-ffa450edef68
Fix default value of sessionIdAttribute.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1527474 13f79535-47bb-0310-9956-ffa450edef68