timer_stats/doc: Fix /proc/timer_stats documentation

Documentation/timers/timer_stats.txt claims:

 "The readout of /proc/timer_stats automatically disables sampling. The sampled
  information is kept until a new sample period is started. This allows multiple
  readouts."

Some simple testing shows that reading from /proc/timer_stats
does not disable sampling. Looking back over past source code,
it appears that it never did.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Link: http://lkml.kernel.org/r/53636FB6.9060407@gmail.com
[ Small tidyup of the changelog. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>