IMO, "release notes" are the part of documentaions which lists known bugs/issues of the product. All the minor and not-so-minor flaws which there were no resources to fix in that version, but should be fixed in next releases. "RN" should be the summary, with details included in more appropriate part. The lack of unit test to any module should be mentioned here.
In your case, the part of the documentation that describes tests shuold have a discussion of what the functional tests could not cover and why.
There is one more problem you should bear in mind turning down unit tests - any reuse of any part of your code is questionable. It could be easier to write it from scratch than to use yours.
Mind that it is my private opinion of what good "release notes" are.
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