Update dag-run.rst to mention Airflow's support for extended cron syntax through croniter

[Rafnel]

Without diving into the source code, it is unclear to readers that Airflow supports an extended cron syntax due to its usage of croniter. My team spends a lot of time making schedules for our DAGs and I believe it would be helpful to mention this. This documentation change should point readers to croniter for all supported cron formats. Also it provides an example of an extended syntax cron expression which Airflow supports.

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[boring-cyborg]

Congratulations on your first Pull Request and welcome to the Apache Airflow community! If you have any issues or are unsure about any anything please check our Contribution Guide (https://github.com/apache/airflow/blob/main/CONTRIBUTING.rst)
Here are some useful points:

• Pay attention to the quality of your code (ruff, mypy and type annotations). Our pre-commits will help you with that.
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[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.