server: doc: document the `--defrag-thold` option

[jakexcosme]

Note: This issue was copied from ggml-org#6293

Original Author: @phymbert
Original Issue Number: ggml-org#6293
Created: 2024-03-25T06:40:20Z

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Context

The --defrag-thold has been added in:

• server: benchmark: chat/completions scenario and other llm servers comparison ggml-org/llama.cpp#5941 (comment)

But it might be documented in the server README.md