doc: add TLSSocket.isSessionReused() docs

The API has existed forever and is used in our unit tests. It is
supported for TLS1.3 as well as 1.2 and useful for troubleshooting, so
it should be documented.

PR-URL: https://github.com/nodejs/node/pull/25423


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>