doc: update to use NAPI_AUTO_LENGTH

Update so that doc says to use NAPI_AUTO_LENGTH instead
of -1 to indicate null terminate string.

PR-URL: https://github.com/nodejs/node/pull/16187


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>