docs: Fix underspecification and haddock markup of 'group'

Please take a few moments to address the following points:

• ensure that your commits are either individually buildable or squashed

• ensure that your commit messages describe what they do (referring to tickets using #NNNN syntax when appropriate)

• Fixed dodgy haddock markup ('quotes' in a code block).

• Removed unhelpful cycle from example

• Replaced code block with haddock repl syntax

• Replaced cons (:) notation with list ([]) notation

• Fixed underspecification of NonEmpty.group and OldList.group