Responses schema fixes (#3650)

Fixes #2237.

Corrects the response schemas for:

```
PUT /user/{user_id}/account_data/{account_dataType}
PUT /user/{user_id}/rooms/{roomId}/account_data/{type}
PUT /directory/list/room/{roomId}
PUT /sendToDevice/{eventType}/{txnId}
POST /account/3pid
POST /account/3pid/add
POST /account/3pid/bind
```

changelogs/client_server/newsfragments/3650.clarification

@@ -0,0 +1 @@
+Correct the schema for the responses  for various API endpoints.

data/api/client-server/account-data.yaml

@@ -67,6 +67,10 @@ paths:
         200:
           description:
             The account_data was successfully added.
+          examples:
+            application/json: {}
+          schema:
+            type: object
       tags:
         - User data
     get:
@@ -151,6 +155,10 @@ paths:
         200:
           description:
             The account_data was successfully added.
+          examples:
+            application/json: {}
+          schema:
+            type: object
       tags:
         - User data
     get:

data/api/client-server/key_backup.yaml

@@ -298,9 +298,10 @@ paths:
       responses:
         200:
           description: The update succeeded.
+          examples:
+            application/json: {}
           schema:
             type: object
-            properties: {}
         400:
           description: |-
             A parameter was incorrect.  For example, the `algorithm` does not

data/api/client-server/list_public_rooms.yaml

@@ -104,6 +104,8 @@ paths:
           description: The visibility was updated, or no change was needed.
           examples:
             application/json: {}
+          schema:
+            type: object
         404:
           description: The room is not known to the server
           examples:

data/api/client-server/to_device.yaml

@@ -85,7 +85,8 @@ paths:
           description:
             The message was successfully sent.
           examples:
-            application/json: {
-              }
+            application/json: {}
+          schema:
+            type: object
       tags:
         - Send-to-Device messaging