[doc] adds the missing documentation of the `server.method` settings.

TL;DR; For all the issues that comes with HTTP POST I recommend instance
maintainers to switch to GET and lock the property in the preferences:

```yaml
server:
  method: GET

preferences:
  lock:
    - method
```

We don't want this in the defaults of the SearXNG distributions for the pros vs
cons listed in this discussion:

- https://github.com/searxng/searxng/pull/3619

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