We don't have release notes for minor updates. Typically, if you see something with a third digit that's non-zero, it is a branch specifically to support hardware that wasn't ready when the main branch was made -- for example, 8.1.0 was a major OS release, but 8.1.2 was made to support the a number of Roku TV models that were released while 8.1.0 was being finalized. The minor branch is made to avoid destabilizing the builds used for most of the Roku devices with changes for those specific models.
If it's just a build number change, like 8.1.0.4102 to 8.1.0.4143, that's usually just a bug fix, addressing some issue seen during beta testing or during the staged roll out.