Fix /sync example (#2077)

* Fix sync example

The same event should not appear in `state` and in the `timeline` so we cannot use the same event twice.

To provide a `state` example we assume that with lazy-loading the user did not get the state event for `@example:example.org`, so we add one since they sent a message in the timeline.

The events that are referenced include a `room_id`, which doesn't appear on this endpoint, so we copy them without it.

Finally, the `join` event of `@alice:example.org` is wrong because the sender does not match the state key, which wouldn't pass the authorization rules.

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

* Fix the `m.room.member.yaml` example

This is a `join` event, and the `sender` doesn't match the `state_key`, so the event couldn't pass the authorization rules.

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

* Add changelog

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

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

changelogs/client_server/newsfragments/2077.clarification

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+Fix the example of the `GET /sync` endpoint and the `m.room.member` example used in several places.