docs: devlink: clarify the scope of snapshot id

In past discussions Jiri explained snapshot ids are cross-region.
Explain this in the docs.

v3: new patch

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>