docs: add description for polymorphic ts typing (#315)

Author: ymc9

docs/guides/polymorphism.md

@@ -240,6 +240,26 @@ model Video extends Content {
 }
 ```
 
+### Polymorphic TypeScript Types
+
+The TS types generated by Prisma into the `@prisma/client` module have the database representation's typing and do not provide the "polymorphic view". For example, the following code doesn't compile:
+
+```ts
+import type { Prisma } from '@prisma/client';
+
+const where: Prisma.PostWhereInput = {};
+console.log(where.published); // error: Property 'published' does not exist on type 'PostWhereInput'
+```
+
+To use the TS types with polymorphic view, import the `Prisma` type from the `@zenstackhq/runtime/models` module instead. This module contains types generated by ZenStack that comply with the ZModel schema.
+
+```ts
+import type { Prisma } from '@zenstackhq/runtime/models';
+
+const where: Prisma.PostWhereInput = {}
+console.log(where.published); // `PostWhereInput` has the `published` field
+```
+
 ### Sample Project
 
 - Simple TypeScript script sample: [https://github.com/zenstackhq/v2-polymorphism](https://github.com/zenstackhq/v2-polymorphism)