doc: fix out of date napi_callback doc

The earlier version `napi_callback` returns `void` but now is
`napi_value`. The document of this section hasn't been modified.

PR-URL: https://github.com/nodejs/node/pull/13570
Fixes: https://github.com/nodejs/node/issues/12248
Fixes: https://github.com/nodejs/node/issues/13562


Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jason Ginchereau <jasongin@microsoft.com>
Reviewed-By: James M Snell <jasnell@gmail.com>