FastAPI docs on async (#3775)

* FastAPI docs on async

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docs/integrations/fastapi.md

@@ -40,6 +40,22 @@ app = FastAPI()
 app.include_router(graphql_app, prefix="/graphql")
 ```
 
+<Note>
+
+Both FastAPI and Strawberry support sync and async functions, but their behavior
+is different.
+
+FastAPI processes sync endpoints in a threadpool and async endpoints using the
+event loop. However, Strawberry processes sync and async fields using the event
+loop, which means that using a sync `def` will block the entire worker.
+
+It is recommended to use `async def` for all of your fields if you want to be
+able to handle concurrent request on a single worker. If you can't use async,
+make sure you wrap blocking code in a suspending thread, for example using
+`starlette.concurrency.run_in_threadpool`.
+
+</Note>
+
 ## Options
 
 The `GraphQLRouter` accepts the following options: