Turn display fix

When setting a new primary or secondary turn, clear the bits from the currently-set turn first.

This issue occurs when, for example, a lane can go both straight or slight-right, and the required turn is slight-right. This would then cause a sharp-right turn to be displayed. (because the bits are just OR'd together, so you would have `001 | 110 = 111`)

Also, update the comment on the weight between the primary and secondary turns.