doc/go1.18: add docs on Cookie.Valid
doc/go1.18: add docs on Cookie.Valid

Updates #47694

Change-Id: Ibf3e25fbccf82e4abd7bd76b2ea6ceb4b05d4664
Reviewed-on: https://go-review.googlesource.com/c/go/+/368357
Trust: Jeremy Faller <b3f594e10a9edcf5413cf1190121d45078c62290@golang.org>
Run-TryBot: Jeremy Faller <b3f594e10a9edcf5413cf1190121d45078c62290@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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