doc: doc lifetime of n-api last error info

Document the lifetime of the structure returned
by napi_get_last_error_info

Backport-PR-URL: https://github.com/nodejs/node/pull/19447
PR-URL: https://github.com/nodejs/node/pull/13939
Fixes: https://github.com/nodejs/abi-stable-node/issues/251


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Hitesh Kanwathirtha <digitalinfinity@gmail.com>