LU-8616 utils: document specific MDT index selection

LU-8616 utils: document specific MDT index selection

Document the "lfs setdirstripe -i" ability to specify explicit MDT
index values for striped direcotories.

Test-Parameters: trivial
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I23b0b5211677bc935b37ebbbfa0a2999c03ebbe5
Reviewed-on: https://review.whamcloud.com/33216
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>