Documentation for foldBs doesn't really explain anything

[ocharles]

The documentation currently reads as

Yet another type of fold.

Which doesn't help the user at all. We should improve this to explain what the fold does and why it's useful.

In general, I'd like to spend a bit of time improving documentation - will coordinate with @atzeus when he's back to see when is a good time to do this (no point starting if there are big API changes coming!)

[atzeus]

Hi Oliver! I agree that this documentation is not helpful :)

I'd very much appreciate it if you improve the documentation! I've got no big API changes planned. Pull requests welcome :)