It's called a rollover, not a transfer. The "issuing institution" will send you a Form 1099-R for the distribution. After you enter the 1099-R in TurboTax it will ask you a series of questions. One of the questions asks whether your moved the money to another retirement account. When you say that you did, it will ask you if you rolled it over. If you say that you rolled it over, TurboTax will indicate on your tax return that it was a rollover. That is the only explanation that is needed. It's up to you to make sure that you meet all the requirements for a rollover. The fact that you complete the rollover in the next year doesn't matter. Don't rush through the questions in TurboTax. Read the questions carefully and make sure you answer them accurately.
