LU-13805 llite: Improve sync_io comments

LU-13805 llite: Improve sync_io comments

Correct and improve comments on cl_sync_io_wait_recycle.

Test-parameters: trivial
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I7784aa75df46831d1b501c823ec1ada48376b227
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/50167
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Qian Yingjin <qian@ddn.com>