gutf8: Document that out args from g_utf16_to_utf8() are non-negative
Despite their type, the values returned will always be ≥ 0. It’s unfortunate they weren’t declared with an unsigned type, but we can’t change that now without breaking API.
Spotted in !2294 (merged).
Signed-off-by: Philip Withnall pwithnall@endlessos.org