StateFilter and RoomEventFilter for filter API are duplicates

In the [filter API](https://matrix.org/docs/spec/client_server/r0.6.0#post-matrix-client-r0-user-userid-filter), the `StateFilter` and `RoomEventFilter` types seem to have an exactly identical definition.

Is this intentional (eg. to denote a future semantic difference) or a bug? And if it is intentional, shouldn't it include a note that the two are currently the same, so that an implementer doesn't need to read both and (expect to) spot the differences?

Start with the linked filter API section in the Matrix client-server specification and compare the definitions of StateFilter and RoomEventFilter. Determine whether their identical definitions are intentional; done means either documenting the equivalence clearly or recording the confirmed distinction in the specification.

Written by the indexing model from the issue text.