LUDOC-399 dom: improve description of inode limits

LUDOC-399 dom: improve description of inode limits

Fix one typo in the MDT inode size description (current default is
2KB/inode instead of old 4KB/inode) value.

Clarify the description of DoM usage and limits.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I42259e45fdbfb2e8541745ce4f794c2b123ebbe5
Reviewed-on: https://review.whamcloud.com/33562
Tested-by: Jenkins
Reviewed-by: Stephan Thiell <sthiell@stanford.edu>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Reviewed-by: Joseph Gmitter <jgmitter@whamcloud.com>