html: add Tokenizer.Raw comment re byte offsets

Change-Id: I2a08f28fcc58869b0e8a3b21b9a9c97da5063014
Reviewed-on: https://go-review.googlesource.com/c/net/+/198357
Reviewed-by: David Symonds <dsymonds@golang.org>

html/token.go

@@ -1067,6 +1067,11 @@ loop:
 
 // Raw returns the unmodified text of the current token. Calling Next, Token,
 // Text, TagName or TagAttr may change the contents of the returned slice.
+//
+// The token stream's raw bytes partition the byte stream (up until an
+// ErrorToken). There are no overlaps or gaps between two consecutive token's
+// raw bytes. One implication is that the byte offset of the current token is
+// the sum of the lengths of all previous tokens' raw bytes.
 func (z *Tokenizer) Raw() []byte {
 	return z.buf[z.raw.start:z.raw.end]
 }