Arm backend: Add docstrings for operator_support/pool_2d_support.py

[Sebastian-Larsson]

cc @digantdesai @freddan80 @per @zingo @oscarandersson8218

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/14683

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❌ 2 New Failures, 2 Unrelated Failures

As of commit de56339 with merge base 68b2d3c ():

NEW FAILURES - The following jobs have failed:

• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 41e2fd3a4e39f7858913c2393a2b53237dc72c25c93a51790d157285f1e9b630 /exec failed with exit code 1
• trunk / test-models-linux-aarch64 (mobilebert, portable, linux.arm64.2xlarge) / linux-job (gh)
  RuntimeError: Command docker exec -t 0107d8920bd9d92bf28fd28e084842f3970a002d281d9614deca485b50eb2d3dtime="2025-09-30T06:00:01Z" level=error msg="error waiting for container: context canceled" /exec failed with exit code 1

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

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

• pull / test-binary-size-linux-gcc / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / test-setup-linux-gcc / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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