Mention exact word counts in implementation documentation of List

Summary

Location of documentation issue: the Haddocks of base, precisely the documentation of List.

The documentation includes a section called "Implementation" which shows the memory layout of lists, but it does not mention exactly how much words are taken up by each part.

Proposed improvements or changes

Mention that each box in the diagram is one word, and therefore that the total memory overhead of a fully (spine-) evaluated list of n elements is 3n + 1 words.