update adding columns

Author: isabellaenriquez

develop-docs/backend/application-domains/database-migrations/index.mdx

@@ -440,9 +440,12 @@ With postgres 14, columns can be added to tables of all sizes as deploy time
 migrations if you follow the guidelines on default values & allowing nulls. When
 creating new columns they should either be:
 
-- Not null with a default. https://develop.sentry.dev/database-migrations/#adding-not-null-to-columns
+- Not null with a default
 - Created as nullable. If no default value can be set on the column, then it's best just to make it nullable.
 
+For nullable columns with a constraint and not null columns with a default, see [Adding Constraints to Columns](#adding-constraints-to-columns-including-not-null). For bigger tables, these constraints can lock the table and cause downtime.
+
+
 ### Adding Columns With a Default
 
 Since we run Postgres >= 14 in production we are able to add columns with a default. To do so, instead of using `default=<your_default>`, use `db_default=<your_default>`. This tells Django to set a default at the database level and manage it there, rather than managing it in application code.