[Migration guide] Add note about prefering change_column_null over change_column

doc/development/what_requires_downtime.md

@@ -139,6 +139,8 @@ Adding or removing a NOT NULL clause (or another constraint) can typically be
 done without requiring downtime. However, this does require that any application
 changes are deployed _first_. Thus, changing the constraints of a column should
 happen in a post-deployment migration.
+NOTE: Avoid using `change_column` as it produces inefficient query because it re-defines
+the whole column type. For example, to add a NOT NULL constraint, prefer `change_column_null `
 ## Changing Column Types