Add min_classical_gap, max_graph_size example

[JoelPasvolsky]

@arcondello re: adding an example for every assertion, one point to consider is that the docstring can get very long, see e.g., from_config here https://github.com/dwavesystems/dwave-cloud-client/blob/master/dwave/cloud/client.py, which makes for a lot of scrolling to see beyond it in online help