Patch from Paul Jakma <paulj@alphyra.ie>

Documentation/sysctl/fs.txt is incorrect wrt to the middle number of
/proc/sys/fs/file-nr.  The current docs state it is the number of
in-use file-handles, from observation this number is actually the
number of /unused/ file-handles - we've observe that when it hits 0 we
get file handle problems (rather than when it hits == file-max as docs
imply).