In my experience, once you've got a bit of documentation, finding everything relevant to an intended change can be non-trivial and error-prone even if it's done first.
For documentation to remain relevant there needs to be some kind of process actually checking every part of it against reality.
Plus you cannot really automate checks for documentation up-to-dateness.
I mean, with actual code you get some help from the tools. No silver bullet, but at least you have type checks, compiler errors, something. But with docs there's no way to automate checks to see if the doc is still relevant and accurate. And what terrible tools are there tend to lead to "boilerplate docs", like those javadocs mentioned in a comment elsewhere.
For documentation to remain relevant there needs to be some kind of process actually checking every part of it against reality.