net: document that a keep-alive failure also returns a timeout
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 <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Emmanuel Odeke <277f8eaa8c7dfbc55c17bf5e30561c1a9c45c569@gmail.com>
Reviewed-by: Filippo Valsorda <4f40cda291c5f9634e1affd3db44947af61f705c@golang.org>

golang/go,cloudflare/go,akutz/go,nawawi/go,CAFxX/go,akutz/go,akutz/go,golang/go,CAFxX/go,akutz/go,golang/go,nawawi/go,cloudflare/go,golang/go,golang/go,CAFxX/go,nawawi/go,akutz/go,akutz/go,CAFxX/go,golang/go,nawawi/go,nawawi/go,golang/go,golang/go,akutz/go,nawawi/go,cloudflare/go,cloudflare/go,CAFxX/go,nawawi/go,nawawi/go,CAFxX/go,cloudflare/go,akutz/go,cloudflare/go,CAFxX/go,cloudflare/go,cloudflare/go,CAFxX/go