unsafe: document the behavior of Sizeof on an interface
unsafe.Sizeof() can return a different value than
reflect.TypeOf(x).Size() for a variable of an interface static type.
This change points out the difference in behavior, by emphasizing that
unsafe.Sizeof() only returns the size of the interface value itself,
rather than the size of the value stored in the interface.
Fixes #67465.
Change-Id: Ia6a809debb7970be171b0fc186209e5d161784e7
Reviewed-on: https://go-review.googlesource.com/c/go/+/586275
Auto-Submit: Ian Lance Taylor <iant@golang.org> Reviewed-by: Ian Lance Taylor <iant@google.com> Reviewed-by: Carlos Amedee <carlos@golang.org> Reviewed-by: Felix Geisendörfer <felix.geisendoerfer@datadoghq.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Commit-Queue: Ian Lance Taylor <iant@golang.org>