- make it clear that 3D @module will only work if each fragment is on a different concrete type

Reviewed By: captbaritone

Differential Revision: D66555487

fbshipit-source-id: 86e4cec2c2bca04398bcafbad54a59efff8e861e

Author: lynnshaoyu

website/docs/guides/data-driven-dependencies/client-3d.md

@@ -184,6 +184,10 @@ in this example, these separate fragments are `FOO_FRAGMENT`, `BAR_FRAGMENT`, an
 to this fragment's data as an argument.
 3. Return the final component using Relay's `MatchContainer`, providing the returned query data as a prop.
 
+Notice that in Client 3D, just as in Server 3D, you cannot use `@module` on multiple fragments on the SAME concrete type (but they can be on the same abstract type i.e. a union or an interface).
+
+So in this example, `Client3DFooComponent_Fragment` is on the concrete type `Client3DFoo`, and `Client3DBarComponent_Fragment` is on the concrete type `Client3DBar`. If `Client3DBarComponent_Fragment` was also on `Client3DFoo`, the relay compiler would report an error. However, all three concrete types implement the same parent interface `IClient3D`, which is fine.
+
 <FbInternalOnly>
 
 > **NOTE:** If you are in www, but not in Comet, you should use `RelayFBMatchContainer` instead of `MatchContainer`.

website/docs/guides/data-driven-dependencies/server-3d.md

@@ -276,6 +276,8 @@ public function commentContentRenderer(Traversable<string> $supported): Awaitabl
 
 Your Relay fragment can now use `@match` to specify that for the `comment_content_renderer` field, we expect dependencies to be decided by the data. In this example, if the `comment_content_renderer` field is of type `CommentMarkdownRenderer`, we load the `CommentMarkdownRenderer.react` component and use the `CommentMarkdownRenderer_comment` fragment to load its data. Similar for the plaintext variant.
 
+> **NOTE:** The inline fragments annotated with `@module` on the same parent 3D fragment must be on distinct concrete types. If they are on the same concrete type, the relay compiler will report an error. So in the example below, `CommentMarkdownRenderer_comment` must be on a different concrete type than `CommentPlaintextRenderer_comment` (for example, the former could be on a `MarkdownComment` type, and the latter on a `PlaintextComment` type. Both could implement a parent interface `Comment`).
+
 On the Relay side you'd write:
 
 ```graphql