LUDOC-388 stats: Correct extents_stats information

LUDOC-388 stats: Correct extents_stats information

The Reference section of the Lustre Manual needs
minor corrections in the sections about collecting
statistics. These corrections include setting the
extents_stats parameter to 1 to reset statistics
and specifying what can be used to activate collection
of statistics.

Change-Id: I4b3f91476427f6be581634c383daebd8b2c8acda
Signed-off-by: James Nunez <james.a.nunez@intel.com>
Reviewed-on: https://review.whamcloud.com/28450
Tested-by: Jenkins
Reviewed-by: Joseph Gmitter <joseph.gmitter@intel.com>