Add note about singularities for polarTheta and polarPhi #2185

Job	Run time
NAMD (main) / test_backends	7m 53s
LAMMPS (develop) / test_backends	11m 5s
GROMACS (main) / test_backends	15m 0s
VMD / test_backends	2m 5s
	36m 3s

Provide feedback