Add docstrings for `ParamViewer` and `_TensorViewer`

[lhenkelm]

When writing a custom modifier, users need to deal with correctly indexing into the model parameter tensor to pick up the correct parameters for their custom modifier instances. ParamViewer and _TensorViewer already exist and might help with that task, but lack docstrings explaining their usage and behavior.

Related: #1641

Discussed in https://github.com/scikit-hep/pyhf/discussions/1716

^{Originally posted by lhenkelm December 1, 2021}

• Not a question, but docstrings for ParamViewer and _TensorViewer (and its associated factory methods) would be very useful.