Added advise for num_workers=0 in docs/speed

[marcm-ml]

Advise for disabling warning message for dataloaders with intentional low workers

What does this PR do?

Closes #10182 by adding recommendation when users specifically set num_workers=0. User are invited to filter out the low worker UserWarning in https://github.com/PyTorchLightning/pytorch-lightning/blob/c33df2639f19d49c5e7520294e3221efe402d684/pytorch_lightning/trainer/data_loading.py#L109-L115

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[carmocca]

Great!