Clarify why it's a good thing that variables of builtin types reject subtypes.
Clarify why it's a good thing that variables of builtin types reject subtypes.

da-woods/cython,cython/cython,cython/cython,cython/cython,scoder/cython,scoder/cython,cython/cython,da-woods/cython,da-woods/cython,da-woods/cython,scoder/cython,scoder/cython