LU-930 doc: update lfs-setstripe.1 man page

LU-930 doc: update lfs-setstripe.1 man page

Update the lfs-setstripe.1 man page to describe
maximum value for the mirror_count option.

Test-Parameters: trivial
Signed-off-by: Alex Deiter <adeiter@tintri.com>
Change-Id: I1879eabef5c397730e0795858e6c6a103d6a2259
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/53672
Tested-by: Maloo <maloo@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>