Edit discussions API annotation

This adds the parameter annotations to be included in autogenerated grape-swagger OpenAPI documentation. Previously, there was no distinction between different noteables, so all noteables were assigned the same parameters and types even though they differ among noteables (e.g. https://docs.gitlab.com/ee/api/discussions.html#create-new-merge-request-thread). This adds some logic to ensure the autogenerated documentation makes those distinctions.

The OpenAPI V2 documentation will be used for dogfooding API fuzzing as per #372422 (closed) and the OpenAPI autogeneration &8926.