Arm backend: Add docstrings for bmm and conv2d operators

[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/14461

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❌ 3 New Failures

As of commit bab309c with merge base 7e228ee ():

NEW FAILURES - The following jobs have failed:

• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 1d495e5a9ba194be18503e5d6d0223aeaf4c41b2762b007795a0fb8dbb6ad873 /exec failed with exit code 1
• pull / unittest-editable / macos / macos-job (gh)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_mv3_model
• trunk / test-arm-ootb-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 87ad66c0da4644f094a499a147d7bcf42b0404ab5ada7bc42abdedca46a7afae /exec failed with exit code 1

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[digantdesai]

#14466 fixes it?

[zingo]

Fails are unrelated arm-oobto... is fixed by another PR