[doc] Update quantization instructions for clarity

[mergennachin]

4bit quantization was a bit buried down, during the hackathon

[pytorch-bot]

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❌ 5 New Failures, 6 Pending, 2 Unrelated Failures

As of commit da813c1 with merge base 0a1dfb2 ():

NEW FAILURES - The following jobs have failed:

• Build Presets / linux (llm, linux.2xlarge, executorch-ubuntu-22.04-clang12) / build (gh)
• pull / android / run-emulator (gh)
  The process '/usr/bin/sh' failed with exit code 255
• pull / test-vulkan-models-linux / linux-job (gh)
  RuntimeError: Data processing error: CAS service error : ReqwestMiddleware Error: Request failed after 5 retries
• pull / unittest-arm-backend-with-no-fvp (test_pytest_models) / linux-job (gh)
  RuntimeError: Command docker exec -t ebacbae09aa8810fbcb81bb6e1b8b66ed2c29d3f5e6ff0816211f8ca7b309234 /exec failed with exit code 1
• pull / unittest-nxp-neutron / linux-job (gh)
  RuntimeError: Command docker exec -t d56f57ad093e1e1dc7f685523e9f583890937b241d197aacf88115bfc52ff7ce /exec failed with exit code 1

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / unittest / linux / linux-job (gh) (matched linux rule in flaky-rules.json)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.

BROKEN TRUNK - The following job failed but was present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest-editable / linux / linux-job (gh) (trunk failure)

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