Document the additive behavior of `rgblink -S/--scramble`

RGBLINK's `-S/--scramble` option lets you specify it multiple times, passing each section type's scramble spec in a different option. For example, `rgblink -S romx=16 -S wramx=4`.

A subsequent option that specifies the same section type(s) as a previous option will warn about overriding, just like we usually do when the same option is repeated. But the additive behavior for different types is useful and worth documenting as intentional.