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GH-33973: [Python][Docs] Update documentation for Parquet filter keyword (#33974)

### Rationale for this change

I wrote a converter from an arbitrary expression to DNF, but this was not needed after learning that it just accepts an expression now.

Closes #33973

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