database/sql: add doc comment for ErrTxDone

Change-Id: Idffb82cdcba4985954d061bdb021217f47ff4985
Reviewed-on: https://go-review.googlesource.com/29850Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/database/sql/sql.go

@@ -1224,6 +1224,8 @@ type Tx struct {
 	}
 }
 
+// ErrTxDone is returned by any operation that is performed on a transaction
+// that has already been committed or rolled back.
 var ErrTxDone = errors.New("sql: Transaction has already been committed or rolled back")
 
 func (tx *Tx) close(err error) {