Add note about QueuedQuery.Exec and BatchResults.Close

[WGH-]

When reviewing my own old code, I noticed several monstrosities like this:

batchResult := tx.SendBatch(ctx, batch)
defer batchResult.Close()
for range batch.Len() {
        if _, err := batchResult.Exec(); err != nil {
                return err
        }
}
return nil

All of them can replaced with just this:

return tx.SendBatch(ctx, batch).Close()

So I thought it might be a good idea to give a hint in the docs explicitly. This trick is not so apparent, after all.

[jackc]

LGTM