Back in September 2015, Terry Myerson (Executive Vice President, Windows and Devices Group) stated in a Privacy and Windows 10 blog:
You are in control with the ability to determine what information is collected.

(Except that you're not...)

He went on to state:
This includes data like an anonymous device ID, device type, and application crash data which Microsoft and our developer partners use to continuously improve application reliability. This doesn’t include any of your content or files, and we take several steps to avoid collecting any information that directly identifies you, such as your name, email address or account ID. (My emphasis)

Around the same time a ZDNET article - No, Microsoft is not spying on you with Windows 10 - claimed the same and implied that anyone concerned about the telemetry (and privacy implications) was - basically - nuts.

However, a subsequent ZDNET article in Feb. 2016 reversed the previous stance and admitted Win 10's telemetry settings can transmit personal info "especially at the higher telemetry settings"... and advised 'dialing back' the settings. Note that at the time there were 3 settings... there are now only "Basic" and "Full" now that "Enhanced" has been axed.

In addition, the article reports that if telemetry is set to "Full" (i.e. the default) and there is a triggering event and "Microsoft’s privacy governance team" decides that they need to access your data, Microsoft can access "user content, such as documents".

Bear in mind that at the default "Full" setting, you have already granted Microsoft permission to collect extra data. The company doesn't have to contact you to ask your permission to collect additional data... you've already signed that right of privacy away.

More than 2 years later and we still don't have full transparency (IMO), granular controls nor a big red Nope button.