Merge 1536310 from trunk:

* docs/manual/upgrading.xml: Note 2.4 backwards-compat break for
  some piped logging configurations.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1536367 13f79535-47bb-0310-9956-ffa450edef68