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

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• documentation is changed or added
• commit message follows commit guidelines