net/9p: Change linuxdoc names to match functions.

As on Jeopardy, my question is in the form of a patch:  Does this have
some special meaning, or is it an accident?  (I looked at other
filesystems but they didn't bother having doc entries for their
init/exit function that I could find.)

Signed-off-by: Rob Landley <rob@landley.net>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>