Fix mention of foldl' in Haddocks of unions/unionsWith

[amesgen]

Tiny documentation fix, was confused by this for a split second today 😆

Before:

After:

[amesgen]

Another option would be to remove the inlined implementation from the Haddocks all-together, as the up-to-date actual implemenation is just one click away.

[treeowl]

The documentation should have everything a user needs to know to understand both the meaning of the function and its performance. In this case, I think the easiest way to do that is to give the actual implementation.

[treeowl]

In the future, please refrain from including images in issues or issue discussions unless they contribute substantially; they are generally bad for accessibility. A diagram of an idea or a rendering of a formatting change would be fine, but these images weren't helpful.

[konsumlamm]

There's actually another mistake: The functions work on any Foldable, not just lists.

[konsumlamm]

Suggested change

	-- | The union of a list of maps:
	-- | The union of a 'Foldable' of maps:

[treeowl]

I would go with "'Foldable' container".

[konsumlamm]

Suggested change

	-- | The union of a list of maps, with a combining operation:
	-- | The union of a 'Foldable' of maps, with a combining operation:

[amesgen]

@konsumlamm Personally, I have no opinion on whether that is misleading (after all, Foldable can be characterized by toList). In any case, if this change is desired, it should also be changed in Data.Map.Strict.Internal/ Data.Set.Internal/Data.IntSet.Internal, which might or might not be in scope of this PR.