Add documentation about what is expected in request bodies

[d-perl]

E.g. https://diamondlightsource.github.io/blueapi/main/reference/rest-spec.html#post--tasks would have you believe that no parameters are taken for this rest call, but instead the plan name and params should be supplied in the body

[callumforrester]

Strange, it looks like it should include this per the docs.

I think this needs investigating, maybe sphinxcontrib-openapi's flavour of openapi is subtly different than fastapi's