Add docstrings list_ext module

After #172 , we should add docstrings to the list_ext module. I know the functions are pretty self-explanatory, but we need to develop the discipline of good docstrings (like the OCaml manual :smile: )

https://github.com/marigold-dev/deku/pull/172#discussion_r723537786
https://github.com/marigold-dev/deku/pull/172#discussion_r723613631

Locate the list_ext module and review the linked pull request discussions for the expected docstring style. Add docstrings for its functions; the work is done when the module's public functions are documented consistently with the project's OCaml documentation conventions.

Written by the indexing model from the issue text.