add comments on WSS computation

[keiyamamo]

https://github.com/KVSlab/VaMPy/blob/2d1c05d45c83831da3664d708ce8d6b8697b3524/src/vampy/automatedPostprocessing/postprocessing_common.py#L144

Point to make

• works for P1P1, but not P2P1
• p is unused since pressure term disappears in the process of extracting tangential component.

[hkjeldsberg]

@keiyamamo Good point! I have added these points to the class description in #138