Either commit to keeping documentation rigorously up to date, or save yourself the effort to write it. With code it's easy: keep the documentation in the same repository as your code and make documentation an integral part of the review process.
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I would actually suggest the opposite: keep documentation in a different repository so that it's easier to update at any time. Documentation almost always follows code, so when it's in the same repo, it's difficult to hold up a release because the docs aren't ready.
In a separate repo, the code can be released early, and PRs to the docs repo can be merged much more eagerly/optimistically at any time. You can also retroactively document bugs discovered in the code if they're not tied together.