or alternately, treat the documentation as code and don't accept code changes which are not properly documented..
I would barely consider random snips about a thing/keyword, scattered over time, to be documentation.
Certainly, a problem was likely solved with this FYI system... but it wasn't a documentation problem.
I also prefer the 'documentation is code' approach, but it doesn't solve the same problem.
A more robust implementation would parse the code to object-code, then emit a document(s) for the developer to fill in.
The killer feature of such a system is that non-code changes (whitespace, etc.) would not trigger a need to re-document, but seemingly benign changes that affect distant parts of code (or a -O4 being added to the Makefile) would trigger a round of documentation
see also: BSD-derived os's commit history, etc.