LU-5394 doc: Update the routerstat man page

LU-5394 doc: Update the routerstat man page

Correct the description of output values, add
a description of additional lines printed when
providing an interval value, add an example,
and other minor editing.

Signed-off-by: James Nunez <james.a.nunez@intel.com>
Change-Id: I9d6221a5f2d3a164c39d550e635a9fc8d0967b69
Reviewed-on: http://review.whamcloud.com/11345
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Amir Shehata <amir.shehata@intel.com>
Reviewed-by: Richard Henwood <richard.henwood@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>