Add comment on utility of tbsCertificateList.Raw

Signed-off-by: Alexander Scheel <alex.scheel@hashicorp.com>

src/crypto/x509/x509.go

@@ -2264,6 +2264,9 @@ func CreateRevocationList(rand io.Reader, template *RevocationList, issuer *Cert
 	if err != nil {
 		return nil, err
 	}
+
+	// Optimization to only marshal this struct once, when signing and
+	// then embedding in certificateList below.
 	tbsCertList.Raw = tbsCertListContents
 
 	input := tbsCertListContents