LUDOC-11 dne: fix examples for default dir layout

LUDOC-11 dne: fix examples for default dir layout

Improve wording around default directory layouts, and discourage the
use of default striped directories.

Fix the examples given for enabling default directory layouts
during upgrade to actually include the "-D" option.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I13dee2b7fc4d80db4ac9e33a193cc3a4773ebbe5
Reviewed-on: https://review.whamcloud.com/c/doc/manual/+/52552
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>