doc/go1.18: add Conn.NetConn documentation to crypto/tls section

Updates #47694

Change-Id: Iead44baa8b2a06ecf7b92d343ed9117f0fc0793e
Reviewed-on: https://go-review.googlesource.com/c/go/+/368315


Trust: Jeremy Faller <jeremy@golang.org>
Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Jeremy Faller <jeremy@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Agniva De Sarker <agniva.quicksilver@gmail.com>