src: warn about odd UTF-16 decoding function signature

Using a `uint16_t` sequence for UTF-16 processing would typically
imply that the sequence already contains the correct 16-bit
code units.

However, our API does not account for that. The previous comments
were somewhat misleading, since endianness is typically applied to
sequences of bytes, which is not something that this API works with.

PR-URL: https://github.com/nodejs/node/pull/22623


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>