When adding the support for file migration with the file: transport,
we missed adding documentation for it.
Signed-off-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Peter Xu <peterx@redhat.com>
Link: https://lore.kernel.org/r/20240220224138.24759-2-farosas@suse.de
Signed-off-by: Peter Xu <peterx@redhat.com>