unsafe: document that Alignof, Offsetof, and Sizeof return a constant

Nothing is changing but the documentation, which did not mention
this property of these functions.

Fixes #27587.

Change-Id: I75bcee4a1dd9ec8cd82826c9a6e02ba7d599f719
Reviewed-on: https://go-review.googlesource.com/c/141377Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/unsafe/unsafe.go

@@ -178,11 +178,13 @@ type Pointer *ArbitraryType
 // The size does not include any memory possibly referenced by x.
 // For instance, if x is a slice, Sizeof returns the size of the slice
 // descriptor, not the size of the memory referenced by the slice.
+// The return value of Sizeof is a Go constant.
 func Sizeof(x ArbitraryType) uintptr
 
 // Offsetof returns the offset within the struct of the field represented by x,
 // which must be of the form structValue.field. In other words, it returns the
 // number of bytes between the start of the struct and the start of the field.
+// The return value of Offsetof is a Go constant.
 func Offsetof(x ArbitraryType) uintptr
 
 // Alignof takes an expression x of any type and returns the required alignment
@@ -193,4 +195,5 @@ func Offsetof(x ArbitraryType) uintptr
 // within that struct, then Alignof(s.f) will return the required alignment
 // of a field of that type within a struct. This case is the same as the
 // value returned by reflect.TypeOf(s.f).FieldAlign().
+// The return value of Alignof is a Go constant.
 func Alignof(x ArbitraryType) uintptr