If you have unused tuition amounts from previous years that exceed $5000, you can ignore the warning. It's designed to ensure that users have entered accurate amounts.
If you imported your 2016 information into this year's return, your leftover tuition amounts are carried forward automatically. This figure is then used to calculate the proper tuition deduction for 2017, based on your current income and deductions.
No need for additional documentation as CRA already has your tuition balance on record.
