docs/design: add notes about swap algorithms

- Indicate that swap-using-offset is preferred over swap-using-move.
  Reference: https://github.com/mcu-tools/mcuboot/pull/2162#issuecomment-2589210084

- Indicate that swap-using-scratch may be removed in the coming future.
  Reference: https://github.com/mcu-tools/mcuboot/pull/2162#issuecomment-2586442176



Signed-off-by: Diego Herranz <diegoherranz@diegoherranz.com>