Create MR API returns 201 for success

For users replying on response code, the API of creating MR returns
201 for a successful response.

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doc/api/merge_requests.md

@@ -1004,6 +1004,7 @@ order for it to take effect:
    value of zero disables approvals for that project.
 1. The provided value of `approvals_before_merge` must be greater than the
    target project's `approvals_before_merge`.
+1. This API returns 201 (created) for a successful response. 
 
 ```json
 {