Add a disclaimer to `HashSet.toList` that order is unspecified

[andreasabel]

The warning given for HashMap.toList is missing here:

unordered-containers/Data/HashSet/Internal.hs

Lines 441 to 444 in 259dc9e

	-- | \(O(n)\) Return a list of this set's elements. The list is
	-- produced lazily.
	toList :: HashSet a -> [a]
	toList t = Exts.build (\ c z -> foldrWithKey (const . c) z (asMap t))

Note that the order of elements depends on which version of hashable this package is built against, even which minor version (which I find dubious):

• Minor release 1.4.3.0 introduces semantic changes hashable#277