Start splitting up miscellaneous doxygen documentation into separate files.

doxyref.h was created to hold miscellaneous documentation that was not specific
to a part of the code.  This file has grown quite a bit so I decided to start
splitting parts of it out into new files.  Now, you can drop a new file into
include/asterisk/doxygen/ and it will be processed by doxygen.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@186953 65c4cc65-6c06-0410-ace0-fbb531ad65f3