doc: correct tls.rootCertificates to match implementation

Update tls.rootCertificates documentation to clarify that it returns
the bundled Node.js root certificates rather than the root certificates
used by tls.createSecureContext.

Fixes: https://github.com/nodejs/node/issues/32074
Refs: https://github.com/nodejs/node/issues/32229

PR-URL: https://github.com/nodejs/node/pull/33313


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>