n-api: update documentation

Document which APIs work while an exception is pending. This is the
list of all APIs which do not start with `NAPI_PREAMBLE(env)`.

Fixes: https://github.com/nodejs/abi-stable-node/issues/257
Backport-PR-URL: https://github.com/nodejs/node/pull/19447
PR-URL: https://github.com/nodejs/node/pull/19078


Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>