Knockback belongs on axes, not swords

A change to the knockback enchantment, simply makes it so you can only use knockback on an axe instead of a sword, making knockback be the first "axe exclusive enchantment".

Mojang really needs to add more axe enchantments, even if its only cleaving. That way every different weapon has a different version of the "sharpness" enchantment (density, power, impaling, yadayada)