doc/design: Add header for swap using scratch algorithm
doc/design: Add header for swap using scratch algorithm

Added sub-header for point where swap-using-scratch algorithm
is described.
Added note on the maximum image size allowed for this mode.

Signed-off-by: Andrzej Puzdrowski <0222c24e1773dd88269ed0e559909ec9bea3604a@nordicsemi.no>

runtimeco/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot