doc/go1.23: document "net/http".Request.CookiesNamed method

doc/go1.23: document "net/http".Request.CookiesNamed method

CL 511516 added the method but didn't include a release note for it
because it was authored and tested before the new release note flow.

For #61472.

Change-Id: I38f73e97093a2badaea658ed430e174b73e35b3a
Reviewed-on: https://go-review.googlesource.com/c/go/+/571278
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Commit-Queue: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>