Also document the special behavior of Alignof(s.f), and mention the
correspondence between Alignof and reflect.Type.{Align,FieldAlign}.

Change-Id: I6f81047a04c86887f1b1164473225616cae45a26
Reviewed-on: https://go-review.googlesource.com/18949
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>