doc: fix doc for napi_create_function

Merge the two duplicate entries for `napi_create_function()`.

Fixes: https://github.com/nodejs/node/issues/21606
PR-URL: https://github.com/nodejs/node/pull/21627/


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>