[Cherry-Pick for 0.20] Revamp decoding docs (#8633)

[NicolasHug]

Cherry-pick of #8633

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/vision/8666

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❌ 5 New Failures

As of commit 67650a1 with merge base db5f8a0 ():

NEW FAILURES - The following jobs have failed:

• CMake / windows (windows.g5.4xlarge.nvidia.gpu, cuda, 11.8) / windows-job (gh)
  [Errno 13] Permission denied: 'C:\\Jenkins\\Miniconda3\\envs\\ci\\DLLs\\_asyncio.pyd'
• Lint / c-source / linux-job (gh)
  RuntimeError: Command docker exec -t af163c5e1cd7eb855f8eb1b8bd49233a092d1157e56d0ca826fcae8ff69ad398 /exec failed with exit code 1
• Tests / unittests-macos (3.10, macos-m1-stable) / macos-job (gh)
  ##[error]fatal: couldn't find remote ref refs/pull/8666/merge
• Tests / unittests-macos (3.11, macos-m1-stable) / macos-job (gh)
  ##[error]fatal: couldn't find remote ref refs/pull/8666/merge
• Tests / unittests-windows (3.9, windows.g5.4xlarge.nvidia.gpu, cuda, 11.8) / windows-job (gh)
  ##[error]fatal: couldn't find remote ref refs/pull/8666/merge

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