btrfs-progs: docs: clarify swapfile an multi-device filesystem

It is possible to create swapfile on a multi-device filesystem but it's
not reliable. The check that verifies that in kernel:

10698                 } else if (device != map->stripes[0].dev) {
10699                         btrfs_warn(fs_info, "swapfile must be on one device");
10700                         ret = -EINVAL;
10701                         goto out;
10702                 }

This does not count devices but rather the actual placement of the
swapfile extents, so multi-device filesystem with single profile can
create it as long as there's enough space and the allocator decides to
place it properly.

[ci skip]

Pull-request: #839
Signed-off-by: David Sterba <dsterba@suse.com>

Documentation/ch-swapfile.rst

@@ -16,7 +16,11 @@ The limitations come namely from the COW-based design and mapping layer of
 blocks that allows the advanced features like relocation and multi-device
 filesystems. However, the swap subsystem expects simpler mapping and no
 background changes of the file block location once they've been assigned to
-swap.
+swap. The constraints mentioned above (single device and single profile) are
+related to the swapfile itself, i.e. the extents and their placement. It is
+possible to create swapfile on multi-device filesystem as long as the extents
+are on one device but this cannot be affected by user and depends on free space
+fragmentation and available unused space for new chunks.
 
 With active swapfiles, the following whole-filesystem operations will skip
 swapfile extents or may fail: