Improve doxygen comment.

Generate the doc even if build on non-EBCDIC system + fix a comment about non-EBCDIC system. In this case, these vars are undefined, not NULL.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1611908 13f79535-47bb-0310-9956-ffa450edef68