Add to unicode docs

Add text about normalizing non-latin strings in a variety of contexts. Related to #2495.
No. It is related to making one form for characters that can be represented by multiple code sequences in unicode. The point of this line is that taking ascii text and running it through the normalization algorithm doesn't change the text. So `'hi eric!' === unorm.nfd('hi eric!')`. So calling it on text that doesn't need it is not a problem. Further, `unorm.nfd('François Gérard') === unorm.nfd(unorm.nfd('François Gérard'))`.

Perhaps I should make this more explicit here, if it is confusing.