net: document that a keep-alive failure also returns a timeout

Updates #31449

Change-Id: I76490c5e83eb2f7ba529b387a57ba088428aece5
Reviewed-on: https://go-review.googlesource.com/c/go/+/189757


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>