Documentation written inside the code it describes, so the two cannot drift apart. Explanation kept where it belongs.
A modern compound of documentum (Latin, a lesson, proof, from docere, to teach) and streng (Old English, a cord, later a strung-together sequence of characters); explanation carried inside the very thing it explains.
Documentation written where the code it describes actually lives, so the two are less likely to drift apart. A docstring travels with what it explains rather than beside it.