macro that also contained a description for every field, but, that description
was not being used anywhere. This changes it, to make the description available
as a Doxygen comment. Furthermore, it is now easily parsable by Doxygen, while
before it messed up the documentation.


git-svn-id: file:///var/local/repositories/ffmpeg/trunk@8254 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b