LUDOC-500 quota: fix the default quota usage

LUDOC-500 quota: fix the default quota usage

The "-d" short option for "--default" was deprecated in LU-15218
in favor of "-D".  This better matches "lfs setdirstripe -d" and
eventually opens up the use of "-d" for "--delete" in the future.

Document "--default" in the manual since this works with both old
and new Lustre releases.

Fixes: 3bf774f3d9a7 ("LUDOC-500 quota: fix the default quota usage")
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I7a47ad7dd1df4788335d46e98282ed1f9e101f76
Reviewed-on: https://review.whamcloud.com/46606
Tested-by: jenkins <devops@whamcloud.com>