Improve documentation for NumberedObjectCollections on Slicing behavior

[MicahGale]

As brought up in #793, the slicing behavior of NumberedObjecCollection (e.g., Cells, Materials) is not the most intuitive as it's not traditional pythonic. This was done intentionally, however, this behavior is poorly documented.

To resolved this:

1. Update all subclasses with documentation of this in their doc strings, not just in NumberedObjectCollection
2. Clarify this in the tutorial
3. Correct an error in the jupyter tutorials.

[digvijay-y]

Should I take this, too?

[MicahGale]

Sure!