Surface dunder methods in API docs

[microbit-matt-hillsdon]

For e.g. Image the dunder methods explain useful functionality (e.g. Image.HEART - Image.SMALL_HEART; Image.HEART * 0.5). We document some of this in Reference but not in the API documentation.

I'd like us to consider doing this in a way that explains the meaning of the dunder methods. Perhaps the examples added on #709 would be sufficient.

This would also help in some niche cases such as the mem class added on #723 which only has dunder methods.