Add compatibility note.

Turn some tabs into space.
Remove some useless <br /> in <highlight> blocks. (this was causing some additionnal blank lines at the end of some examples)

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1684314 13f79535-47bb-0310-9956-ffa450edef68