Adding documentation to geom_geom_collider about potential future conflicts

[AlpAydinoglu]

We are currently using nhat_BA_W in geom_geom_collider , there is this warning (https://drake.mit.edu/doxygen_cxx/structdrake_1_1geometry_1_1_signed_distance_pair.html):

For two geometries that are just touching (i.e., distance = 0), the underlying code can guarantee a correct value for nhat_BA_W only when one geometry is a sphere, and the other geometry is a sphere, a box, or a cylinder. Otherwise, the underlying code is not in place yet to guarantee a correct value for nhat_BA_W when surfaces are just touching, and the vector will be populated by NaN values.

This should probably be added to as a comment to geom_geom_collider for people who might consider using it with shapes other than spheres.