database/sql: document Result methods

Fixes #5110

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/19280046

src/pkg/database/sql/sql.go

@@ -1637,7 +1637,16 @@ func (r *Row) Scan(dest ...interface{}) error {
 
 // A Result summarizes an executed SQL command.
 type Result interface {
+	// LastInsertId returns the integer generated by the database
+	// in response to a command. Typically this will be from an
+	// "auto increment" column when inserting a new row. Not all
+	// databases support this feature, and the syntax of such
+	// statements varies.
 	LastInsertId() (int64, error)
+
+	// RowsAffected returns the number of rows affected by an
+	// update, insert, or delete. Not every database or database
+	// driver may support this.
 	RowsAffected() (int64, error)
 }