Export a function to append to or override the comment for a Rule

For example,
```haskell
module Test.Cardano.Conway.Cuddle where

[...]

import Test.Cardano.Shelley.Cuddle qualified as Shelley

[...]

transaction_output :: Rule
transaction_output = extendComment "New information introduced in this era..." Shelley.transaction_output

or

transaction_output :: Rule
transaction_output = overrideComment "New comment!" Shelley.transaction_output
```