LU-7209 doc: more accurate documentation for obdfilter-survey

LU-7209 doc: more accurate documentation for obdfilter-survey

Make the the description of obdfilter-survey accurate and
precise.

Signed-off-by: Richard Henwood <richard.henwood@intel.com>
Change-Id: Icdd4adf53643e91dc8a2539f63977aae5fe28fe0
Reviewed-on: http://review.whamcloud.com/16646
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Omkar Kulkarni <omkar.kulkarni@intel.com>
Tested-by: Omkar Kulkarni <omkar.kulkarni@intel.com>
Reviewed-by: Cliff White <cliff.white@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>