Backport r1513461 to fix some Doxygen warnings/comments, except for the...

Backport r1513461 to fix some Doxygen warnings/comments, except for the following files which rely on other patches which have not been backported yet:
   - ap_mpm.h: r1493741 
   - http_log.h: r1512819
   - httpd.h: r1426877
   - mpm_common.h: which is already in synch with 2.4
So only mpm_var_buf.h remains. This is however needed in order to backport other doxygen clean-up.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1611541 13f79535-47bb-0310-9956-ffa450edef68