I want a function (doc) in a type class (Documentable) that works in several monads (M1, M2)

stderr

Test.hs:48:5: error:
    • Ambiguous type variable ‘a0’ arising from a use of ‘doc’
      prevents the constraint ‘(Documentable (M2 a0))’ from being solved.
      Probable fix: use a type annotation to specify what ‘a0’ should be.
      These potential instance exist:
        instance Documentable (M2 ()) -- Defined at Test.hs:31:10
    • In a stmt of a 'do' block:
        doc "Hello, this is a comment in monad M2"
      In the expression:
        do doc "Hello, this is a comment in monad M2"
           m1_in_m2 $ do doc "Hello, this is a comment in monad M1"
      In an equation for ‘foo’:
          foo
            = do doc "Hello, this is a comment in monad M2"
                 m1_in_m2 $ do doc "Hello, this is a comment in monad M1"
   |
48 |     doc "Hello, this is a comment in monad M2"
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Test.hs

{-# Language TypeSynonymInstances #-}
{-# Language FlexibleInstances #-}
{-# Language FlexibleContexts #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}

module Main where

import Control.Monad.Except
import Control.Monad.Trans.State
import Data.Sequence as S

data M1State = M1State
    { docs :: S.Seq String -- Sequence of strings
    } deriving (Eq, Show)

type M1 a = StateT M1State (Except String) a

newtype M2 a = M2 { runM2 :: M1 a } 
    deriving (Monad)

instance Functor M2 where
    fmap = liftM

instance Applicative M2 where
    pure   = return
    (<*>)  = ap

class Documentable a where
    doc :: String -> a

instance Documentable (M2 ()) where
    doc txt = do
        M2 $ doc txt

instance Documentable (M1 ()) where
    doc txt = do
        modify (\s -> s { docs = docs s |> txt })

initialM1 = M1State { docs = S.empty }

runM1 = runExcept . flip execStateT initialM1

m1_in_m2 :: M1 a -> M2 a
m1_in_m2 = M2

foo :: M2 ()
foo = do
    doc "Hello, this is a comment in monad M2"
    m1_in_m2 $ do
        doc "Hello, this is a comment in monad M1"

main = do
    putStrLn "Hello World"