doc/go1.16: document net TCP listener's backlog size update

doc/go1.16: document net TCP listener's backlog size update

Updates #41470

Change-Id: Iebd3a339504aa7f8834853d6a740557fb3bce3ef
Reviewed-on: https://go-review.googlesource.com/c/go/+/262938
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Trust: Emmanuel Odeke <emm.odeke@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>