Move comment from .c to .h in order to make use of it in doxygen.

Fix the comment that states that these vars are NULL on ASCII machine, they seem to be undefined instead.
Rework some comments.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1611916 13f79535-47bb-0310-9956-ffa450edef68