LU-18244 docs: update lfs mkdir description

LU-18244 docs: update lfs mkdir description

Accompany with the patch 56925 where "lfs mkdir -C -N" is
supported aiming to align the behavior of OSTs, this patch
adds related description.

Test-Parameters: trivial
Signed-off-by: Keguang Xu <squalfof@gmail.com>
Change-Id: I17f8aa3d5fa7df2c2b419320a0bedd0686ebaee0
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/57011
Tested-by: Maloo <maloo@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>