Update migration.md (#782)

Co-authored-by: vito_chao <[email protected]>

pages/docs/migration.md

@@ -8,7 +8,7 @@ layout: page
 Automatically migrate your schema, to keep your schema up to date.
 
 {% note warn %}
-**NOTE:** AutoMigrate will create tables, missing foreign keys, constraints, columns and indexes. It will change existing column's type if its size, precision, nullable changed. It **WON'T** delete unused columns to protect your data.
+**NOTE:** AutoMigrate will create tables, missing foreign keys, constraints, columns and indexes. It will change existing column's type if its size, precision changed, or if it's changing from non-nullable to nullable. It **WON'T** delete unused columns to protect your data.
 {% endnote %}
 
 ```go