LUDOC-548 sec: doc update for 'deny_mount' nm property

LUDOC-548 sec: doc update for 'deny_mount' nm property

This patch updates the "Managing the Properties" section of the Lustre
Operations Manual to introduce the 'deny_mount' nodemap property
implemented as part of LU-18469.

Signed-off-by: Marc Vef <mvef@whamcloud.com>
Change-Id: Ib496b84c1b4284c0933afa2ba4fdf3f4da107e8e
Reviewed-on: https://review.whamcloud.com/c/doc/manual/+/58365
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>