Amend API docs to be more precise about invalid input.
Amend API docs to be more precise about invalid input.

Fixes #59
Reported-by: https://github.com/daurnimator

rockdaboot/libpsl,darnir/libpsl,dago/libpsl,dago/libpsl,dago/libpsl,darnir/libpsl,rockdaboot/libpsl,darnir/libpsl,rockdaboot/libpsl