[low-bit optim] Update docs on supported platforms and caveats

[gau-nernst]

Addresses #959, #958, #955

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/ao/971

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

❗ 1 Merge Blocking SEVs

There is 1 active merge blocking SEVs. Please view them below:

• (merge blocking) PyTorch Testing Nodes Undergoing ROCm 6.2.1 Upgrades

If you must merge, use @pytorchbot merge -f.

✅ You can merge normally! (2 Unrelated Failures)

As of commit cb1dc15 with merge base 96e8fee ():

BROKEN TRUNK - The following jobs failed but were present on the merge base:

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

• Run Regression Tests / test (CPU Nightly, linux.4xlarge, --pre torch --index-url https://download.pytorch.org/whl/nightl... / linux-job (gh) (trunk failure)
• Run Regression Tests / test (CUDA Nightly, linux.g5.12xlarge.nvidia.gpu, --pre torch --index-url https://download.pytorc... / linux-job (gh) (trunk failure)

This comment was automatically generated by Dr. CI and updates every 15 minutes.