Document the behavior of fromListWith with non-associative and non-commutative operations

[Lysxia]

Also copy the improvements of #133 from Strict to Lazy.

On top of f newVal oldVal, we also need to explain that the elements are accumulated left-to-right. The fact that fromListWith (++) gives the elements in reverse order seems somewhat counter-intuitive to me, but it is what it is.

[emilypi]

For reference, this is what's produced:

This looks fine to me, unless there's a preference for fancydoc styles.

[sjakobi]

Cheers! This is very valuable documentation!

My suggestions below are optional, since this is already a large improvement on the status quo.

[sjakobi]

BTW, CI has been fixed in the meantime, so a rebase should take care of the red status here.

[Lysxia]

Thanks for the reviews :)

[Lysxia]

I made the two existing examples more concrete. The last one is tricky to improve because it would be best illustrated with a non-associative operation, and a good instance is hard to come by.

[sjakobi]

Thanks, that's a great concrete example! :)

I think that the "gotcha" documented here is a bit too important to be hidden within the collapsible Examples section though. You could add a second heading to end the collapsible section, or alternatively just remove the collapsible section heading.

Apologies for the endless bikeshedding BTW! ;)

[Lysxia]

Fair enough!

[sjakobi]

Thanks again, @Lysxia! :)