It's written in laTex, and the source files for the manual are in the main SVN sources. You can simply check out a copy and submit a patch to the patch tracker like you would submit a patch for the actual program.
If you can't manage latex, write up a text document explaining its use, what section you'd put it in, etc, for the manual, and post that to the patch tracker (this is likely to take longer to get included because someone else will have to go in and adapt it for the actual manual).
But either way, the manual gets generated from source in the Manual folder in the SVN sources, and to submit changes to it simply use the same patch tracker.
