doc: update who to cc for n-api

Add info about who to @cc for issues with n-api

PR-URL: https://github.com/nodejs/node/pull/13335
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jason Ginchereau <jasongin@microsoft.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Hitesh Kanwathirtha <digitalinfinity@gmail.com>

doc/onboarding-extras.md

@@ -23,6 +23,7 @@
 | `lib/util` | @bnoordhuis, @cjihrig, @evanlucas |
 | `lib/zlib` | @addaleax, @bnoordhuis, @indutny |
 | `src/async-wrap.*` | @trevnorris |
+| `src/node_api.*` | @nodejs/n-api |
 | `src/node_crypto.*` | @nodejs/crypto |
 | `test/*` | @nodejs/testing |
 | `tools/eslint`, `.eslintrc` | @not-an-aardvark, @silverwind, @trott |