Wait for your Form W-2 and Form 1099-R.
You'll enter your 2019 Form W-2 as exactly received, presumably with the code D amount of $21,000 in box 12.
I assume that the return of excess contribution was processed in 2019 and you'll be receiving a 2019 Form 1099-R near the end of January. You'll enter this code 8 2019 Form 1099-R exactly as received. Presumably this will show $2,100 in box 1 and $100 in box 2a [corrected previous typo]. If so, you'll also need to enter $2,000 as Other income not already reported on a Form W-2 or Form 1099-R. TurboTax will add the $2,000 and $100 to your wages on line 1 of Form 1040.
TurboTax will not ask you to provide any explanation. It wouldn't hurt to separately prepare an explanation statement and include it with your mailed tax return, but you could just omit the explanation, e-file, and respond to the IRS with the explanation if they ever ask for it.
