[Feature Request]: Document that file-based sinks return a PCollection of output filenames

### What would you like to happen?

It appears that the filebased sinks, such as `WriteToText` output a PCollection that is a list of files: https://github.com/apache/beam/blob/36486447e4d07af5076830ca1e331a6b61f14986/sdks/python/apache_beam/io/filebasedsink.py#L355

This is not clear from the documentation, for example, the docstring on WriteToText doesn't clarify the nature of output PCollection in https://beam.apache.org/releases/pydoc/2.45.0/apache_beam.io.textio.html#apache_beam.io.textio.WriteToText .

### Issue Priority

Priority: 2 (default / most feature requests should be filed as P2)

### Issue Components

- [X] Component: Python SDK
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Start with sdks/python/apache_beam/io/filebasedsink.py around line 355 and the WriteToText docstring in apache_beam.io.textio. Clarify in the relevant documentation that file-based sinks return a PCollection containing output filenames, then check the generated Python API documentation to confirm the behavior is visible.

Written by the indexing model from the issue text.