doc: clarify use of NAPI_EXPERIMENTAL

We've had a few questions about APIs not being available which
were related to not having specified `NAPI_EXPERIMENTAL`.

Add some additional documentation to explain this common
issue:

Refs: https://github.com/nodejs/node-addon-api/issues/810



Signed-off-by: Michael Dawson <mdawson@devrus.com>

PR-URL: https://github.com/nodejs/node/pull/35195


Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Gabriel Schulhof <gabriel.schulhof@intel.com>
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>