staging/lustre: Add documentation for unstable_stats in sysfs

commit ac5b1481 ("staging: lustre: osc: Track and limit
"unstable" pages") added a new sysfs variable, but corresponding bit of
documentation was not forgotten.

Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>