Improve doxygen comment.

Improve layout, fix typo, generate relevant doc even if build on non-EBCDIC system + add note that these functions are mostly noop on non-EBCDIC system.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1611858 13f79535-47bb-0310-9956-ffa450edef68