MINOR: list: documenting mt_list_for_each_entry_safe() macro

MINOR: list: documenting mt_list_for_each_entry_safe() macro

- Adding some comments in mt_list_for_each_entry_safe() macro to make it
  somehow understandable.
  The macro is performing critical stuff but was not documented at all.
  Moreover, nested loops with conditional tricks are used,
  making it even harder to understand the steps performed in it.

- Updating mt_list_for_each_entry_safe usage example.

- Added a "FIXME:" comment in a specific condition that seems to
  never be reached even when deeply stress-testing mt_lists
  (using test_list binary provided in the repository).