docs(migrations): add example for conditional unique index (#12578)

Valetek
2020-08-01T19:05:34-03:00

Co-authored-by: Valentin Almeida--Lemaire &lt;valentin.almeida@onrewind.com&gt;
Co-authored-by: Pedro Augusto de Paula Barbosa &lt;papb1996@gmail.com&gt;