Document that btrfs replace wipes magic string in superblocks of the original FS

[werman]

At the moment documentation reads as:

On a live filesystem, duplicate the data to the target device which is currently stored on the source device. If the source device is not available anymore, or if the -r option is set, the data is built only using the RAID redundancy mechanisms. After completion of the operation, the source device is removed from the filesystem. If the srcdev is a numerical value, it is assumed to be the device id of the filesystem which is mounted at path, otherwise it is the path to the source device. If the source device is disconnected, from the system, you have to use the devid parameter format. The targetdev needs to be same size or larger than the srcdev.

The only clue that something may happen to original FS is "the source device is removed from the filesystem", which doesn't sound too alarming for someone not well versed in the relevant terminology. In fact it happened to me when I tried to use btrfs replace to copy data to a new ssd. I was able to recover old FS with https://superuser.com/questions/1281942/recover-data-from-replaced-btrfs-disk/1715352#1715352 though.

It would be great if documentation would state it a clear language that old FS won't be usable after btrfs replace.

It also would be great to have #534 idea implemented.