Clarify unsafety

`unsafeEncodeUtf` only throws a (pure) exception on invalid input. This is not unsafe in the same way as e.g. `unsafePerformIO`, which can cause undefined behaviour. Other functions that also throw pure exceptions (like `head`) are documented as partial (which is more correct in my opinion).

`unsafeFromChar` only truncates its input. In what way is this unsafe? Can it cause undefined behaviour?

I think the documentation for these functions should be clarified.