Keeping documentation in another document would see this going out of sync over time until the document doesn't at all reflect how the code looks. Not updating anything at all will leave others clueless of why the change was needed 4 months later and might see the other classes grow to the same size. Thus, eventually the code base will once again be opaque unless you ask the nearest person that worked on it last.

A nicer way of handling the flow of the document would be to use a literate programming tool that allows for keeping code snippets in an order which makes sense for the documentation rather than sprinkling as comments between the code. That way, code review happens at the same time as documentation review with code being local to the text documenting it in a format which is easy to understand for both.