chore(docs): Improve data types callout (#126)

Part of celest-dev/celest#54

pages/docs/functions/data-types.mdx

@@ -10,11 +10,10 @@ import { Callout } from 'nextra/components'
 With Celest Functions, you can use any of the core Dart types available such as `int`, `String`, and `DateTime`. You can also use your custom data types. Celest will handle the transfer and formatting of data from your Flutter app to your backend, also known as serialization, out-of-the-box in most cases. In situations requiring custom serialization, you can write your logic which will be used instead.
 
 <Callout type="info">
-Celest does not support the following Dart types as parameter or return types. `Object` and `dynamic` may only be used as the value type of a `Map`.
+Celest does not support the following Dart types as parameter or return types, or as a field of a parameter or return type: 
+`Stream`, `Object`, `dynamic`
 
-- `Stream`
-- `Object`
-- `dynamic`
+To represent raw JSON types, use the `JsonValue` type from `package:celest` instead of `Object?` or `dynamic`.
 </Callout>
 
 # Custom data type example