9p: Update documentation to add fscache related bits

Update the documentation to describe FS-Cache related
caching parameters. This patch also updates the pointers
to 9p-related papers and adds pointer to the Wiki.

Signed-off-by: Abhishek Kulkarni <adkulkar@umail.iu.edu>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>