Add changelog

Signed-off-by: Kévin Commaille <zecakeh@tedomum.fr>

changelogs/client_server/newsfragments/2277.clarification

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+The optional `submit_url` response parameter of the `/requestToken` endpoints uses the same request
+and response parameters and error codes as the Identity Service API's `POST /_matrix/identity/v2/validate/email/submitToken`,
+as per [MSC4183](https://github.com/matrix-org/matrix-spec-proposals/pull/4183).

changelogs/identity_service/newsfragments/2277.clarification

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+Clarify the error codes that can be returned with a 400 HTTP status code by the `POST /_matrix/identity/v2/validate/email/submitToken`
+and `POST /_matrix/identity/v2/validate/msisdn/submitToken` endpoints, introducing the `M_TOKEN_INCORRECT`
+error code, as per [MSC4183](https://github.com/matrix-org/matrix-spec-proposals/pull/4183).