btrfs-progs: Add device management related paragraph.

Add device management related paragraph to better explain btrfs device
management.

Cc: Marc MERLIN <marc@merlins.org>
Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
btrfs-progs: Add explain on btrfs-zero-log.

Add more explain on btrfs-zero-log about when to use it.

Reviewed-by: Marc MERLIN <marc@merlins.org>
Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>