Add support for showing both the normal method def and the pretty syntax

[eric-wieser]

As suggested by @jag1g13 on slack:

For me, some hybrid of __len__() and len(self) feels right. I (as an experienced Python developer) would be potentially confused by anything other than __len__ (is it an actual method called len ?), whereas I would expect novice developers to prefer len(self), since that's the bit they're familiar with