doc: fix doc for napi_get_value_string_utf8

The API for napi_get_value_string_utf8() appears to have been
previously changed. This improves the doc reflect the current design.

PR-URL: https://github.com/nodejs/node/pull/14529
Fixes: https://github.com/nodejs/node/issues/14398


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>