gh-111140: Improve PyLong_AsNativeBytes API doc example & improve the test (#115380)

This expands the examples to cover both realistic use cases for the API.
    
I noticed thing in the test that could be done better so I added those as well: We need to guarantee that all bytes of the result are overwritten and that too many are not written.  Tests now pre-fills the result with data in order to ensure that.

Co-authored-by: Steve Dower <steve.dower@microsoft.com>