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.

Start in python/datafusion/unparser.py and read the existing Dialect and Unparser docstrings, then inspect the current SELECT 1 unparser test for setup. Add doctest-style examples showing the dialect differences, clarify the Dialect and with_pretty summaries, and run pytest with --doctest-modules. Done means all listed public functions have passing usage examples and the wording is corrected.

索引モデルが issue の本文から書いたものです。