[tacostakohashi]

My take on this is that documentation should live in the same repo as the code it relates to.

Depending on the language, you might want to "build" the documentation static html or PDF from markdown or javadoc or whatever, as part of the normal build process.

That way, documentation lives under the same versioning regime as the code. Make a branch to change the behavior... document it in the same branch, merge it together. Check out a version from 3 years ago... you get the matching documentation for 3 years at a time.

Any other "out of band" / wiki based system will always inevitably get out of date, or have awkward references to the "old" and "new" behavior that only loosely correspond to specific versions.