Add ThinGraph example to Multi*Graph doc_strings

[dschult]

Based on discussion in #5813 we should add an example of a subclass of the graph classes to the MultiGraph and MultiDiGraph classes. The ThinGraph example from the Graph and DiGraph doc_strings was suggested in that discussion and would be easy to copy over.

[rossbar]

Agreed - this is a good idea. Another benefit of moving this is that the ThinGraph would actually be executable, which would serve as additional, practical testing of the factory function mechanism for subclasses.