Add example and improve docs for EitherOrBoth::reduce

[2ndDerivative]

Also changed "product" to avoid confusion with multiplication.

This was to address the confusion that led to PR #690 which can be closed in my opinion if this change is added instead. Hit me up for any feedback!

[2ndDerivative]

I added another signature because I commited from a different machine than usual, please just squash these three commits

[phimuemue]

Thanks for this.

bors r+

[bors[bot]]

Build succeeded:

• bors build finished