doc,test: mention Duplex support for TLS

Document and test the existing support for generic Duplex streams
in the TLS module.

PR-URL: https://github.com/nodejs/node/pull/17599


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>