Document a config option properly

Adding a setting that someone will have to tune under pressure.

Document the configuration option below.

Cover, in this order:
- **Name**, type, and the exact place it is set: file, environment variable, flag, or all three, with precedence stated
- **Default**, and why that default was chosen
- **What it controls**, described so a reader knows what will observably change
- **Valid range or values**, and what happens at the extremes
- **When you would change it**: at least two concrete situations, with the direction to move it
- **Interactions**: other options this affects or depends on
- **When it takes effect**: startup only, hot reload, per request
- **Failure mode**: what a wrong value does, whether it fails loudly at startup or misbehaves silently later

If the honest answer is "most people should never change this", write that as the first line.

Option:

Reference and API docs. The lookup layer: function docs, config options, error codes and examples worth copying.