The `limited` flag in `Timeline` in `/sync` response needs clarification

[Link to `/sync` spec.](https://spec.matrix.org/v1.8/client-server-api/#get_matrixclientv3sync)

It currently says:

> True if the number of events returned was limited by the limit on the filter.

However, there are also the following cases:

- When newly joining a room. Though this arguably is covered by the above, we should explicitly state it.
- When there is a gap in the timeline. See https://github.com/matrix-org/synapse/issues/16463

Generally, `limited` means that there is more events the client can request with `/messages`

Start with the linked Matrix `/sync` specification and review the `limited` definition in the Timeline response. Check how the wording covers newly joined rooms, timeline gaps, and fetching more events with `/messages`; the documentation is done when these cases and the general meaning are stated unambiguously.

Written by the indexing model from the issue text.