unsafe: document that Sizeof includes field alignment

Fixes #52018

Change-Id: I6d06d5b5279b9bdc899b0ad43488577d3c0b94be
Reviewed-on: https://go-review.googlesource.com/c/go/+/397516
Trust: Ian Lance Taylor <[email protected]>
Reviewed-by: Emmanuel Odeke <[email protected]>
Reviewed-by: Rob Pike <[email protected]>

Author: ianlancetaylor

src/unsafe/unsafe.go

@@ -187,6 +187,7 @@ 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.
+// For a struct, the size includes any padding introduced by field alignment.
 // The return value of Sizeof is a Go constant.
 func Sizeof(x ArbitraryType) uintptr