LU-14867 doc: a few words about an asterisk in lfs quota

LU-14867 doc: a few words about an asterisk in lfs quota

Clarify the difference between an asterisk printed per OST
in verbouse lfs quota output and an asterisk near the whole
filesystem usage.

Change-Id: I778fe1f7b1f6f8d55c311d81bd2b311d82463390
Test-Parameters: trivial
Signed-off-by: Sergey Cheremencev <sergey.cheremencev@hpe.com>
Reviewed-on: https://review.whamcloud.com/44357
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Petros Koutoupis <petros.koutoupis@hpe.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>