Fix a lot of doxygen warnings. Thanks to Brad Hards for the patch.

I added a few more fixes, and there are still more that might
need a doxygen expert.

PR: 48061
Submitted by: Brad Hards
Reviewed by: poirier


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@830527 13f79535-47bb-0310-9956-ffa450edef68