Add plain comment that will not be processed by Doxygen, which
explains that the list of #defines is intentionally presented as a
nested list.
 
* subversion/include/svn_config.h(Configuration Defines): Add plain
    comment.

Approved by: C. Michael Pilato

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1461180 13f79535-47bb-0310-9956-ffa450edef68