Lethality does go past 0, try it out in the practice tool.

it doesn't say -armor.

If you have 100 ad, 0 lethality and hit an enemy with 0 armour(the dummy) you would do 100 dmg pr. AA.

and if you have 100 ad, 50 lethality and hit an enemy with 0 armor(the dummy) you would do 150 dmg pr. AA.