LU-16598 osp: cleanup comment in osp_sync.c

LU-16598 osp: cleanup comment in osp_sync.c

The comment of osp_sync.c is outdated can be cleaned
up a little bit for better explanation of the implementation.

Signed-off-by: Li Xi <lixi@ddn.com>
Change-Id: I60b39ab5f7360521258200cf55e5c85373cf4aa2
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/50146
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Sergey Cheremencev <scherementsev@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>