✨ Generate correct OpenAPI docs for responses with no content (#621)

docs/tutorial/response-status-code.md

@@ -22,6 +22,10 @@ It will:
 
 <img src="/img/tutorial/response-status-code/image01.png">
 
+!!! note
+    Some response codes (see the next section) indicate that the response does not have a body.
+
+    FastAPI knows this, and will produce OpenAPI docs that state there is no response body.
 
 ## About HTTP status codes
 
@@ -34,11 +38,12 @@ These status codes have a name associated to recognize them, but the important p
 
 In short:
 
-* `100` and above are for "Information". You rarely use them directly.
+* `100` and above are for "Information". You rarely use them directly.  Responses with these status codes cannot have a body.
 * **`200`** and above are for "Successful" responses. These are the ones you would use the most.
     * `200` is the default status code, which means everything was "OK".
     * Another example would be `201`, "Created". It is commonly used after creating a new record in the database.
-* `300` and above are for "Redirection". 
+    * A special case is `204`, "No Content".  This response is used when there is no content to return to the client, and so the response must not have a body.
+* **`300`** and above are for "Redirection".  Responses with these status codes may or may not have a body, except for `304`, "Not Modified", which must not have one.
 * **`400`** and above are for "Client error" responses. These are the second type you would probably use the most.
     * An example is `404`, for a "Not Found" response.
     * For generic errors from the client, you can just use `400`.