LU-930 doc: include status in "lfs df" man page

LU-930 doc: include status in "lfs df" man page

Describe the "DRSI" target states in the lfs-df.1 man page.

Improve the overall description of how "lfs df [-i]" works, with
estimation of total/free inodes for ZFS, and show actual output.

Test-Parameters: trivial
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: I2ae4190c9ebd3467b5243df89938501d301cab07
Reviewed-on: https://review.whamcloud.com/31998
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Joseph Gmitter <joseph.gmitter@intel.com>
Reviewed-by: James Nunez <james.a.nunez@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>