Documentation is available as part of the Linux kernel documentation.
(http://git.kernel.org/?p=linux/kernel/git/ericvh/v9fs.git;a=blob_plain;f=Documentation/filesystems/9p.txt;hb=v9fs-devel)

v9fs development repositories are maintained on kernel.org using the git (http://git.or.cz/) version control system. The primary development repository is the v9fs-devel branch.
(http://git.kernel.org/?p=linux/kernel/git/ericvh/v9fs.git;a=shortlog;h=v9fs-devel).

v9fs development is primarily coordinated via our developer's mailing list (http://sourceforge.net/mail/?group_id=40382), and bug reports should be cc:'d there. There is also a user's mailing list for more general questions about using 9p under Linux.

Compiled kernel-doc source code documentation is available at http://www.kernel.org/~ericvh/kernel-doc with doxygen call-graphs and data structure visualizations available at http://www.kernel.org/~ericvh/doxygen