Merge r1031731 from trunk:

PR#40047: explain that AJP_* variables can be used to send atrributes to the backend, like JkEnvVar.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@1031733 13f79535-47bb-0310-9956-ffa450edef68