docs: clarify Gunicorn's role in webserver worker refresh

[0BVer]

This update revises the documentation for worker_refresh_batch_size to clearly state:

"Number of workers to refresh at a time through Gunicorn's built-in worker management. When set to 0, worker refresh is disabled. When nonzero, Airflow periodically refreshes webserver workers by bringing up new ones and killing old ones."

The change aims to eliminate ambiguity by clarifying that Gunicorn reload and webserver refresh refer to the same underlying mechanism. Previously, these terms could be misinterpreted as distinct functions or mistaken for auto_refresh_interval, leading to confusion. By emphasizing that both phenomena are part of Gunicorn's worker management strategy, users will have a clearer understanding of how Airflow manages webserver workers.

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[boring-cyborg]

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[0BVer]

I applied pre-commit

[boring-cyborg]

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