crypto/tls: set Deadline before sending close notify alert

This change also documents the need to set a Deadline before
calling Read or Write.

Fixes #31224

Change-Id: I89d6fe3ecb0a0076b4c61765f61c88056f951406
Reviewed-on: https://go-review.googlesource.com/c/go/+/266037


Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Katie Hockman <katie@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>