Document when a numpy array is copied and when an error is triggered

[lorentzenchr]

#2455 (comment) already discusses and answers the question when an array passed as argument is copied.
It would be really nice to have it documented somewhere in https://pybind11.readthedocs.io/en/stable/advanced/pycpp/numpy.html:

• When exactly is a new array (with different type) created for an array passed as argument?
• When is an error raised for an array that does not meet the type specified (i.e. difference of passing and not passing py::array::forcecast).