Document EVFILT_FS and VQ_*

Hi,

For a projects I needed to use EVFILT_FS and saw that it hasn't been documented
so here is a patch to document it http://planet.etoilebsd.net/kqueue.2.diff

It is based on the commit message for the implementation of EVFILT_FS, sorry I
don't know how to better document it
(http://lists.freebsd.org/pipermail/cvs-src/2005-September/052288.html)

While using it I also discover that VQ_MOUNT, VQ_UMOUNT, VQ_NOTRESP and
VQ_NOTRESPLOCK are not documented either but I don't know where to document
them.

It would be great to see all of this documented, it would be useful at least for
me  :)

regards,
Bapt