Document additional guarantee of Arg's Hashable instance

Fixes haskell-unordered-containers#242.

Author: josephcsible

src/Data/Hashable/Class.hs

@@ -906,6 +906,8 @@ instance Hashable a => Hashable (Max a) where
     hashWithSalt p (Max a) = hashWithSalt p a
 
 -- | __Note__: Prior to @hashable-1.3.0.0@ the hash computation included the second argument of 'Arg' which wasn't consistent with its 'Eq' instance.
+-- Additionally, as of @hashable-1.3.0.0@, and going forward as part of the API guarantee, the hash of @'Arg' a _@ is the same value as the hash of @a@.
+-- This means that it's safe to add and remove 'Arg' from elements stored in hash-indexed data structures without changing any hashes.
 --
 -- @since 1.3.0.0
 instance Hashable a => Hashable (Arg a b) where