doc: fix return results description of 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.
Fixes: https://github.com/nodejs/node/issues/14398
Checklist
-
make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines
Affected core subsystem(s)
doc, n-api