doc: link n-api module init to multi-load addons

Add a blurb to the documentation of `NAPI_MODULE_INIT()` pointing to
the discussion of the consequences of providing an addon that can be
loaded multiple times from multiple contexts.

PR-URL: https://github.com/nodejs/node/pull/21891


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>