I think thar the most valuable documentation are the commit comments for the version control system you use.
Especially if one uses the incremental iteration paradigm, the commit history together with the code diff IS the documentation of one's code in it's most fundamental way, that is when I changed something, why, what I was ( thinking that I was ) doing etc.
In case that one has to maintain existing code, of course, the whole history is not available but still, refactoring committed similarly documents the evolution of the code in a similar way.
Especially if one uses the incremental iteration paradigm, the commit history together with the code diff IS the documentation of one's code in it's most fundamental way, that is when I changed something, why, what I was ( thinking that I was ) doing etc.
In case that one has to maintain existing code, of course, the whole history is not available but still, refactoring committed similarly documents the evolution of the code in a similar way.