Document `initializedcheck` compiler directive.

Document the `initializedcheck` compiler directive. With the default value of `True` the following is inserted into the generated .c files whenever memoryviews are accessed:

```
if (unlikely(!__pyx_v_lightfm->model_biases.memview)) 
{PyErr_SetString(PyExc_AttributeError,"Memoryview is not initialized");
{__pyx_filename = __pyx_f[0]; __pyx_lineno = 91; __pyx_clineno = __LINE__; goto __pyx_L1_error;}}
```

As far as I can tell this is not documented anywhere at the moment. There is some discussion here: https://mail.python.org/pipermail//cython-devel/2012-April/002289.html

Can anybody comment on the performance impact on changing this?