docs: Add docstring examples to the unparser module

[ting-hong-shieh]

Which issue does this PR close?

Closes #1691.

Rationale for this change

AGENTS.md requires every public Python function to carry a usage example, and
no public function in python/datafusion/unparser.py had one. The omission is
most costly in this module: the reason Dialect exists is that dialects render
the same plan differently, and nothing in the documentation showed that.

What changes are included in this PR?

• Docstring examples for Dialect.default, Dialect.mysql, Dialect.postgres,
  Dialect.sqlite, Dialect.duckdb, Unparser.plan_to_sql and
  Unparser.with_pretty. All build the same input table so the examples differ
  only in the behaviour being demonstrated, and each shows the actual quoting a
  dialect produces:

  >>> Unparser(Dialect.default()).plan_to_sql(plan)
  'SELECT t.a FROM t'
  >>> Unparser(Dialect.mysql()).plan_to_sql(plan)
  'SELECT `t`.`a` FROM `t`'
  >>> Unparser(Dialect.postgres()).plan_to_sql(plan)
  'SELECT "t"."a" FROM "t"'
  

• Corrected the Dialect class summary, which read "DataFusion data catalog."

• Described what with_pretty does — it drops redundant parentheses and returns
  the same unparser so the call can be chained — instead of restating its name.

No behaviour is changed; the diff is docstrings only.

Are there any user-facing changes?

Documentation only. The new examples run as doctests under the existing
--doctest-modules setting, so they are checked in CI.