enhance unetr docstring with examples

### Discussed in https://github.com/Project-MONAI/MONAI/discussions/2547

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<sup>Originally posted by **jhkang0526** July  7, 2021</sup>
hello,

I recently saw about the UNETR. 

Is it currently possible to apply to 3D data?

If possible, how should I use it?

Thanks,

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