Define the semantics for state events sent in a thread

It seems currently to be undefined what should happen if somebody sends a state event with an `m.thread` relationship type.

Obviously the room state will be updated (because servers will not care about the `m.relates_to`), but I'm concerned that clients might choose to hide the state transition in a thread, which would be confusing at best.

Start by reviewing the Matrix specification's existing definitions for state events and the `m.thread` relationship. Determine and document the intended semantics for a state event carrying that relationship, including how clients should represent the resulting state transition; the issue is complete when the specification clearly resolves this ambiguity.

Written by the indexing model from the issue text.