docs: Improve clarity of "Output Only" fields in Dataset class

This commit updates the docstrings for the `Dataset` class to more clearly indicate fields that are "Output Only".

Changes include:
- Adding an "Output only. " marker at the beginning of the docstring (after the type hint) for all server-populated fields such as `id`, `creationTime`, `lastModifiedTime`, `etag`, etc.
- Updating the `Dataset.__init__` docstring to explicitly mention that these "Output Only" fields are populated by the server and require a call to `Client.get_dataset()` to be retrieved by the SDK.
- Ensuring internal helper attributes (`_PROPERTY_TO_API_FIELD` and `_READ_ONLY_PROPERTIES`) are correctly updated to reflect these output-only properties.

These changes aim to reduce confusion for you by aligning the Python SDK documentation more closely with the BigQuery API reference regarding which fields can be set by the client versus which are returned by the server.