btrfs-progs: docs: update note about device deletion

User Kasijjuf points out the VFS initialism is not explained anywhere.
While this could be fixed, the whole note about inability to delete the
device by which the filesystem has been mounted, is wrong.

Issue: #49
Signed-off-by: David Sterba <dsterba@suse.com>

Documentation/btrfs-device.asciidoc

@@ -72,8 +72,9 @@ using the RAID1 profile. See the example section below.
 +
 The operation can take long as it needs to move all data from the device.
 +
-NOTE: It is not possible to delete the device that was used to mount the
-filesystem. This is a limitation given by the VFS.
+It is possible to delete the device that was used to mount the filesystem. The
+device entry in mount table will be replaced by another device name with the
+lowest device id.
 
 *delete* <dev>|<devid> [<dev>|<devid>...] <path>::
 Alias of remove kept for backward compatibility