Add to unicode docs

Add text about normalizing non-latin strings in a variety of contexts. Related to #2495.
I read the doc you linked.  It read to me like normalization can also handle differences between unicode strings, eg where 2 glyphs represent one character vs 1:1.  

> Unicode Normalization Forms are formally defined normalizations of Unicode strings which make it possible to determine whether any two Unicode strings are equivalent to each other.

> The Unicode Standard defines two equivalences between characters: canonical equivalence and compatibility equivalence.  ... **Compatibility equivalence is a weaker equivalence between characters or sequences of characters that represent the same abstract character, but may have a different visual appearance or behavior. **

It reads to me like both ASCII and Unicode encodings could be transformed during normalization.

Anyway, encoding always makes my head hurt.  If you think it's clear, then go for it.