Add some complexities to Data.List documentation (#15003)
Add some complexities to Data.List documentation (#15003)

Namely for:
- head
- uncons
- tail
- last
- init
- null

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc