docs: Update transactions how to example (#1775)

Author: jasonblanchard

docs/howto/transactions.md

@@ -1,32 +1,40 @@
 # Using transactions
+In the code generated by sqlc, the `WithTx` method allows a `Queries` instance to be associated with a transaction.
 
+For example, with the following SQL structure:
+
+`schema.sql`:
 ```sql
 CREATE TABLE records (
   id SERIAL PRIMARY KEY,
   counter INT NOT NULL
 );
+```
 
+`query.sql`
+```sql
 -- name: GetRecord :one
 SELECT * FROM records
 WHERE id = $1;
-```
 
-The `WithTx` method allows a `Queries` instance to be associated with a transaction.
+-- name: UpdateRecord :exec
+UPDATE records SET counter = $2
+WHERE id = $1;
+```
 
+And the generated code from sqlc in `db.go`:
 ```go
-package db
+package tutorial
 
 import (
 	"context"
 	"database/sql"
 )
 
-type Record struct {
-	ID      int
-	Counter int
-}
-
 type DBTX interface {
+	ExecContext(context.Context, string, ...interface{}) (sql.Result, error)
+	PrepareContext(context.Context, string) (*sql.Stmt, error)
+	QueryContext(context.Context, string, ...interface{}) (*sql.Rows, error)
 	QueryRowContext(context.Context, string, ...interface{}) *sql.Row
 }
 
@@ -38,43 +46,34 @@ type Queries struct {
 	db DBTX
 }
 
-func (*Queries) WithTx(tx *sql.Tx) *Queries {
-	return &Queries{db: tx}
+func (q *Queries) WithTx(tx *sql.Tx) *Queries {
+	return &Queries{
+		db: tx,
+	}
 }
 
-const getRecord = `-- name: GetRecord :one
-SELECT id, counter FROM records
-WHERE id = $1
-`
-
-func (q *Queries) GetRecord(ctx context.Context, id int) (Record, error) {
-	row := q.db.QueryRowContext(ctx, getRecord, id)
-	var i Record
-	err := row.Scan(&i.ID, &i.Counter)
-	return i, err
-}
 ```
 
-With pgx you'd use it like this for example:
+You'd use it like this:
 
 ```go
-func bumpCounter(ctx context.Context, p *pgx.Conn, id int) error {
-	tx, err := db.Begin(ctx)
+func bumpCounter(ctx context.Context, db *sql.DB, queries *tutorial.Queries, id int32) error {
+	tx, err := db.Begin()
 	if err != nil {
 		return err
 	}
 	defer tx.Rollback()
-	q := db.New(tx)
-	r, err := q.GetRecord(ctx, id)
+	qtx := queries.WithTx(tx)
+	r, err := qtx.GetRecord(ctx, id)
 	if err != nil {
 		return err
 	}
-	if err := q.UpdateRecord(ctx, db.UpdateRecordParams{
+	if err := qtx.UpdateRecord(ctx, tutorial.UpdateRecordParams{
 		ID:      r.ID,
 		Counter: r.Counter + 1,
 	}); err != nil {
 		return err
 	}
 	return tx.Commit()
 }
-```
+```