database/sql: improve documentation for closing prepared statement on Tx

[thiagoretondar]

The https://golang.org/pkg/database/sql/#Tx documentation says:

The statements prepared for a transaction by calling the transaction's Prepare or Stmt methods are closed by the call to Commit or Rollback.

But in the example of https://golang.org/pkg/database/sql/#Tx.Prepare there's a comment ("Prepared statements take up server resources and should be closed after use.") that can cause some doubts about how to handle it

package main

import (
	"context"
	"database/sql"
	"log"
)

var (
	ctx context.Context
	db  *sql.DB
)

func main() {
	projects := []struct {
		mascot  string
		release int
	}{
		{"tux", 1991},
		{"duke", 1996},
		{"gopher", 2009},
		{"moby dock", 2013},
	}

	tx, err := db.Begin()
	if err != nil {
		log.Fatal(err)
	}
	defer tx.Rollback() // The rollback will be ignored if the tx has been committed later in the function.

	stmt, err := tx.Prepare("INSERT INTO projects(id, mascot, release, category) VALUES( ?, ?, ?, ? )")
	if err != nil {
		log.Fatal(err)
	}
	defer stmt.Close() // Prepared statements take up server resources and should be closed after use.

	for id, project := range projects {
		if _, err := stmt.Exec(id+1, project.mascot, project.release, "open source"); err != nil {
			log.Fatal(err)
		}
	}
	if err := tx.Commit(); err != nil {
		log.Fatal(err)
	}
}

[toothrot]

/cc @bradfitz @kardianos

I believe the documentation is more relevant when preparing a statement outside of a transaction, even though this example is specifically for (*Tx).Prepare(). Noting that they use additional resources on the server, even within the scope of a transaction, seems relevant to me.