At the moment, Microsoft has yet to post a change log on the Windows 10 Mobile update history page, but that will change soon enough. Fast Ring members of the Windows Insider program just got a new 10586.107 build, so we should see a list of what's new and improved on that update soon.
Both this page and the Windows 10 update history page shows that Microsoft is making good on their plans to offer better communications with their customers on what's contained in the new versions of its operating systems.
Thanks to MohammadMSFT for the tip!
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