Document PropType usage with the Composition API (#2157)

Author: skirtles-code

src/guide/typescript/composition-api.md

@@ -131,6 +131,49 @@ export default defineComponent({
 })
 ```
 
+### Complex prop types
+
+With type-based declaration, a prop can use a complex type much like any other type:
+
+```vue
+<script setup lang="ts">
+interface Book {
+  title: string
+  author: string
+  year: number
+}
+
+const props = defineProps<{
+  book: Book
+}>()
+</script>
+```
+
+For runtime declaration, we can use the `PropType` utility type:
+
+```ts
+import type { PropType } from 'vue'
+
+const props = defineProps({
+  book: Object as PropType<Book>
+})
+```
+
+This works in much the same way if we're specifying the `props` option directly:
+
+```ts
+import { defineComponent } from 'vue'
+import type { PropType } from 'vue'
+
+export default defineComponent({
+  props: {
+    book: Object as PropType<Book>
+  }
+})
+```
+
+The `props` option is more commonly used with the Options API, so you'll find more detailed examples in the guide to [TypeScript with Options API](/guide/typescript/options-api.html#typing-component-props). The techniques shown in those examples also apply to runtime declarations using `defineProps()`.
+
 ## Typing Component Emits {#typing-component-emits}
 
 In `<script setup>`, the `emit` function can also be typed using either runtime declaration OR type declaration: