Document "longpolling" and other techniques to handle the 180s backend timeout.

[judu]

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Describe the content you'd like to see

In the "troubleshooting" section, we should address the 180s backend timeout that sōzu enforces. We need to explain this timeout and document the various solutions, would a customer want to "bypass" this timeout:

• Long polling (does it really work?)
• Asynchronous worker (reply immediately that the work is in progress and have the client check for progress afterwards)
• Purchase a dedicated load balancer with different timeouts.