Update backends-coreml.md

[pytorchbot]

Adds more context on quantization to address #12059.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/12117

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❌ 1 New Failure, 4 Unrelated Failures

As of commit 53b5d6f with merge base de24489 ():

NEW FAILURE - The following job has failed:

• pull / unittest-buck / macos / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 3

FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

• pull / test-openvino-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.
• 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 jobs failed but were present on the merge base:

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

• pull / unittest / macos / macos-job (gh) (trunk failure)
  backends/xnnpack/test/ops/test_exp.py::TestExp::test_fp16_exp
• pull / unittest-editable / macos / macos-job (gh) (trunk failure)
  backends/xnnpack/test/ops/test_exp.py::TestExp::test_fp16_exp

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