LU-10885 docs: note flock now being enabled by default

LU-10885 docs: note flock now being enabled by default

mount -o flock was made the default, but the mount.lustre(8) man-page
still said noflock is default.  Text based on comments in LU-10885 and
http://wiki.lustre.org/Mounting_a_Lustre_File_System_on_Client_Nodes.

Signed-off-by: Laura Hild <lsh@jlab.org>
Change-Id: I48bfc0260fb948771f5cf4fb8cbc6ee9588e2217
Test-Parameters: trivial
Fixes: 16fb13eb3863 ("LU-10885 llite: enable flock mount option by default")
Fixes: 3613af3e15cb ("LU-10885 llite: enable flock mount option by default")
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/51948
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Timothy Day <timday@amazon.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>