Missing docstring examples in the `unparser` module

## Description

`AGENTS.md` requires every public Python function to carry a usage example, but
none of the public functions in `python/datafusion/unparser.py` have one:

* `Dialect.default`
* `Dialect.mysql`
* `Dialect.postgres`
* `Dialect.sqlite`
* `Dialect.duckdb`
* `Unparser.plan_to_sql`
* `Unparser.with_pretty`

The gap matters more here than in most modules, because the reason `Dialect`
exists is that different dialects render the same plan differently, and nothing
in the current documentation shows that difference. The existing test unparses
`SELECT 1`, which is identical across all four dialects.

Two smaller documentation problems sit in the same file:

* the `Dialect` class summary reads `"DataFusion data catalog."`
* `with_pretty` is documented as `"Set the pretty flag."`, which does not say
what the flag changes.

## Expected Behavior

All public functions in the module carry doctest-style usage examples,
consistent with the rest of the package and executed by pytest's
`--doctest-modules`.

## Suggested Improvement

Add examples that unparse the same plan through each dialect so the differences
are visible, and correct the two docstrings noted above.

Comece em python/datafusion/unparser.py e leia as docstrings existentes de Dialect e Unparser; em seguida, inspecione o teste atual do unparser para SELECT 1 para entender a configuração. Adicione exemplos no estilo doctest mostrando as diferenças entre os dialetos, esclareça os resumos de Dialect e with_pretty e execute pytest com --doctest-modules. Está concluído quando todas as funções públicas listadas tiverem exemplos de uso aprovados e a redação tiver sido corrigida.

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