LUDOC-460 quota: statfs functionality for project quota

LUDOC-460 quota: statfs functionality for project quota

Document the project quota functionality for "df"
and "lfs df" when being run on a directory with project
quota assigned. This will return the amount of space
available to that project rather the total filesystem
space, if the project quota limit is smaller.

Signed-off-by: Wang Shilong <wshilong@ddn.com>
Change-Id: I33097e508fd9801be63f6aeaa8fa1c6ea586a5ea
Reviewed-on: https://review.whamcloud.com/41412
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>