LU-18110 doc: improve lfs-flushctx manual page

LU-18110 doc: improve lfs-flushctx manual page

There's some pretty strange wording around kerberos/keyring
right now, so let's try to improve it?

Test-Parameters: trivial
Change-Id: Ie905b5b9db54597ad96e46a429fb163f963ff02b
Signed-off-by: Oleg Drokin <green@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/55958
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>