n-api: document that native strings are copied

Mention that a copy is made of the native string by the
napi_create_string_* APIs.

Fixes: https://github.com/nodejs/abi-stable-node/issues/304
PR-URL: https://github.com/nodejs/node/pull/20425


Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>