openapi: Move stray get messages description into OpenAPI.

templates/zerver/api/get-messages.md

@@ -25,10 +25,6 @@
 
 ## Response
 
-When a request is successful, this endpoint returns a dictionary
-containing the following (in addition to the `msg` and `result` keys
-present in all Zulip API responses).
-
 {generate_return_values_table|zulip.yaml|/messages:get}
 
 {generate_response_description(/messages:get)}

zerver/openapi/zulip.yaml

@@ -4223,8 +4223,12 @@ paths:
               schema:
                 allOf:
                   - $ref: "#/components/schemas/JsonSuccessBase"
-                  - $ref: "#/components/schemas/SuccessDescription"
                   - additionalProperties: false
+                    description: |
+                      When a request is successful, this endpoint returns a dictionary
+                      containing the following (in addition to the `msg` and `result` keys
+                      present in all Zulip API responses).
+                      A typical successful JSON response may look like
                     properties:
                       result: {}
                       msg: {}