Clarify intent of flaky test for test_rigid_rotation

[jlevin]

See test_rigid_rotation_with_reflection() in test_rigid_transform.py

This test produces different answers on numpy 1.19.3 and higher on certain architectures. This is because there are duplicate singular values.

Since a reflection through the origin in 3-space is not a rotation at all, and there is no rotation matrix which can reflect through the origin, it's not clear what the correct result should be.

See also numpy/numpy#22945

Runtime information:

NumPy 1.19.3
Python 3.9.13 (main, May 27 2022, 22:45:39)
[GCC 9.4.0]

Note: This behavior is also occurring with numpy 1.24.1
Context for the issue:

The behavior is observed in CircleCI with the following info:
Docker with Python using cimg/python:3.9.13
Model name: Intel(R) Xeon(R) Platinum 8124M CPU @ 3.00GHz

Not reproducible on MacOS or Linux native.
Not reproducible on MacOS or Linux with same Docker image.
Not reproducible in Numpy 1.19.2.