Add reference to a recent FrozenGen PR #149 in the changelog

Also fixup example with `FrozenGen` instance in haddock

Author: lehins

CHANGELOG.md

@@ -1,11 +1,12 @@
 # 1.3.0
 
-* Move `thawGen` from `FreezeGen` into the new `ThawGen` type class. Fixes an issue with
-  an unlawful instance of `StateGen` for `FreezeGen`.
-* Add `modifyGen` and `overwriteGen` to the `FrozenGen` type class
-* Add `splitGen` and `splitMutableGen`
-* Switch `randomM` and `randomRM` to use `FrozenGen` instead of `RandomGenM`
-* Deprecate `RandomGenM` in favor of a more powerful `FrozenGen`
+* Improve `FrozenGen` interface: [#149](https://github.com/haskell/random/pull/149)
+  * Move `thawGen` from `FreezeGen` into the new `ThawGen` type class. Fixes an issue with
+    an unlawful instance of `StateGen` for `FreezeGen`.
+  * Add `modifyGen` and `overwriteGen` to the `FrozenGen` type class
+  * Add `splitGen` and `splitMutableGen`
+  * Switch `randomM` and `randomRM` to use `FrozenGen` instead of `RandomGenM`
+  * Deprecate `RandomGenM` in favor of a more powerful `FrozenGen`
 * Add `isInRange` to `UniformRange`: [#78](https://github.com/haskell/random/pull/78)
 * Add default implementation for `uniformRM` using `Generics`:
   [#92](https://github.com/haskell/random/pull/92)

src/System/Random/Stateful.hs

@@ -795,6 +795,9 @@ applyTGen f (TGenM tvar) = do
 -- Here is an example instance for the monadic pseudo-random number generator
 -- from the @mwc-random@ package:
 --
+-- > import qualified System.Random.MWC as MWC
+-- > import qualified Data.Vector.Generic as G
+--
 -- > instance (s ~ PrimState m, PrimMonad m) => StatefulGen (MWC.Gen s) m where
 -- >   uniformWord8 = MWC.uniform
 -- >   uniformWord16 = MWC.uniform
@@ -804,8 +807,11 @@ applyTGen f (TGenM tvar) = do
 --
 -- > instance PrimMonad m => FrozenGen MWC.Seed m where
 -- >   type MutableGen MWC.Seed m = MWC.Gen (PrimState m)
--- >   thawGen = MWC.restore
 -- >   freezeGen = MWC.save
+-- >   overwriteGen (Gen mv) (Seed v) = G.copy mv v
+--
+-- > instance PrimMonad m => ThawedGen MWC.Seed m where
+-- >   thawGen = MWC.restore
 --
 -- === @FrozenGen@
 --