LU-930 man: add proper documentation for replace_nids command

LU-930 man: add proper documentation for replace_nids command

The current entry in the lctl.8 man page and manual entry are totally
lacking in explanation of what the various NIDs mean.
It should explain the behaviour of failover NIDs.

A separate "lctl replace_nids" man page was created and some
additional information added.

Test-Parameters: trivial
Signed-off-by: Artem Blagodarenko <ablagodarenko@ddn.com>
Change-Id: I35e8fa26c109811a7411a73cd40ad811c2256e1b
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/57270
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Frederick Dilger <fdilger@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>