The answer is generally, you don't. Instead, you print your supporting documents and keep them with the printed copy of your return, in case anyone asks.
Are you amending the return because of this reason for sale? If so, in the amending process (1040X), you will have the opportunity to describe your reason in text. It's not the same as attaching documents, but the IRS is already so overwhelmed with documents, they will thank you for not attaching something they did not ask for.
By the way, where does the IRS ask for reason for sale?
The IRS does not want supporting documents or explanations that are not required by IRS rules. The IRS instructions for Form 1040-X say "Don't attach a copy of your original return, correspondence, or other items unless required to do so." If the IRS has a question about your amended return or wants more information, they will send you a letter asking for it.