gh-128319: Add missing server class attributes to socketserver docs

[stephen-hansen]

Just doing some cleanup of the socketserver docs regarding class attributes as mentioned in the linked issue.

• Move address_family attribute from instance attributes to class attributes.
• Document allow_reuse_port.
• Document max_packet_size (note UDP only).
• Add note to request_queue_size to denote TCP only.

• Issue: Some server class attributes are missing in socketserver docs #128319

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📚 Documentation preview 📚: https://cpython-previews--128320.org.readthedocs.build/

[ZeroIntensity]

Thanks for doing this :)