I think I'll leave this as properties -- I think in most cases this is preferred.

Agreed about the docstrings.

I realise we can't protect much, but I think this was a particularly surprising one, mainly because I think users are surprised that `Path.unit_circle()` is a singleton.  I could have also resolved this by having those return a copy, but that breaks some optimizations in markers and elsewhere where it tries to save the same path only once wherever possible.