database/sql: add note to Scanner that the database owns values

It was unclear that users must copy values out of the src value
for value types like []byte.

Fixes #24492

Change-Id: I99ad61e0ad0075b9efc5ee4e0d067f752f91b8fa
Reviewed-on: https://go-review.googlesource.com/108535Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/database/sql/sql.go

@@ -301,6 +301,10 @@ type Scanner interface {
 	//
 	// An error should be returned if the value cannot be stored
 	// without loss of information.
+	//
+	// Reference types such as []byte are only valid until the next call to Scan
+	// and should not be retained. Their underlying memory is owned by the driver.
+	// If retention is necessary, copy their values before the next call to Scan.
 	Scan(src interface{}) error
 }