Additionally! By default only some enemies have the potential to become legendary enemies by default. You can now customize the Legendary Rate to change what percentage of enemies can become legendary. The default value is 1%. You can now finally see multiple legendary enemies within the same group!
Legendary Rarity affects how many enemies have the "chance" to be legendary. By default, this is placed at 1%. They are added to a metaphorical pool of "potential legendaries."
Legendary Spawn rate affects how likely it is that individual enemies in that pool of potential legendaries will be actual legendaries.
Changing Legendary Rarity will have a greater effect, since by increasing the "pool" of potential legendary enemies you are drastically increasing your chances of encountering legendary enemies. If you just change Legendary Spawn Rate, you're still stuck with only 1% of enemies in the pool in the first place, and therefore only 1% of them are affected by said modified spawn rate.
So go easy on changing Legendary Rarity!