LU-18110 doc: improve wording in lctl-snapshot* manpages

LU-18110 doc: improve wording in lctl-snapshot* manpages

There's some consistently strangely spelled umount when the unmount
action is meant (i.e. not part of the command itself) and other
clearing of the language

Test-Parameters: trivial
Change-Id: If20bcad739e3eb1367f2e028ac45d60d2986fc34
Signed-off-by: Oleg Drokin <green@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/55956
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>