doc: update limitation about transaction isolation level (#398)

Author: ymc9

docs/reference/limitations.md

@@ -13,6 +13,24 @@ This section lists the current limitations of ZenStack.
 
 As a workaround, use [interactive transactions](https://www.prisma.io/docs/concepts/components/prisma-client/transactions#interactive-transactions) instead.
 
+### Minimum transaction isolation level
+
+To ensure access policies are properly enforced, the database's transaction isolation level should be set to at least [Repeatable Read](https://en.wikipedia.org/wiki/Isolation_(database_systems)#Repeatable_reads). This can be done by changing the settings of the database, or providing the `isolationLevel` option when creating `PrismaClient`:
+
+```ts
+const prisma = new PrismaClient({
+  transactionOptions: {
+    isolationLevel: Prisma.TransactionIsolationLevel.RepeatableRead,
+  },
+});
+```
+
+If you don't want to change the global settings, alternatively you can set the `transactionIsolationLevel` option when calling ZenStack's `enhance` API. All the transactions initiated internally by ZenStack will use the specified isolation level.
+
+```ts
+const db = enhance(prisma, { user }, { transactionIsolationLevel: 'RepeatableRead' });
+```
+
 ### MongoDB is not supported
 
 Right now, the focus of this project is SQL databases, and there's no plan to support MongoDB in the near future.