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GH-34098: [Python][Docs] Fix dataset docstring (#34099)

Many `Dataset.from_{table,fragment,batches}` don't have a docstring, or they refer to `Scanner.from_table`. It would be better to plug in the actual docstring in there.

* Closes: #34098

Authored-by: Fokko Driesprong <fokko@tabular.io>
Signed-off-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>